fix(tests/tox): updated gevent in tox config (#4020) (#4037) (#4100)

## Description
Fixed gevent error in tox tests:
```
Collecting gevent<20.10,>=20.9
  Using cached gevent-20.9.0.tar.gz (5.8 MB)
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'error'
  error: subprocess-exited-with-error

  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [60 lines of output]
      warning: src/gevent/resolver/cares.pyx:38:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/resolver/cares.pyx:40:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/resolver/cares.pyx:41:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      Compiling src/gevent/resolver/cares.pyx because it changed.
      [1/1] Cythonizing src/gevent/resolver/cares.pyx
      warning: src/gevent/libev/corecext.pyx:325:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/libev/corecext.pyx:783:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/libev/corecext.pyx:785:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/libev/corecext.pyx:787:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      warning: src/gevent/libev/corecext.pyx:791:0: The 'DEF' statement is deprecated and will be removed in a future Cython version. Consider using global variables, constants, and in-place literals instead. See cython/cython#4310
      Compiling src/gevent/libev/corecext.pyx because it changed.
      [1/1] Cythonizing src/gevent/libev/corecext.pyx
      Compiling src/gevent/_greenlet_primitives.py because it changed.
      [1/1] Cythonizing src/gevent/_greenlet_primitives.py
      Compiling src/gevent/_hub_primitives.py because it changed.
      [1/1] Cythonizing src/gevent/_hub_primitives.py
      Compiling src/gevent/_hub_local.py because it changed.
      [1/1] Cythonizing src/gevent/_hub_local.py
      Compiling src/gevent/_waiter.py because it changed.
      [1/1] Cythonizing src/gevent/_waiter.py
      warning: src/gevent/_gevent_cgreenlet.pxd:112:33: Declarations should not be declared inline.

      Error compiling Cython file:
      ------------------------------------------------------------
      ...
      cdef load_traceback
      cdef Waiter
      cdef wait
      cdef iwait
      cdef reraise
      cpdef GEVENT_CONFIG
            ^
      ------------------------------------------------------------

      src/gevent/_gevent_cgreenlet.pxd:181:6: Variables cannot be declared with 'cpdef'. Use 'cdef' instead.
      Compiling src/gevent/greenlet.py because it changed.
      [1/1] Cythonizing src/gevent/greenlet.py
      Traceback (most recent call last):
```

## Reviewer Checklist
- [ ] Title is accurate.
- [ ] Description motivates each change.
- [ ] No unnecessary changes were introduced in this PR.
- [ ] PR cannot be broken up into smaller PRs.
- [ ] Avoid breaking [API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces) changes unless absolutely necessary.
- [ ] Tests provided or description of manual testing performed is included in the code or PR.
- [ ] Release note has been added for fixes and features, or else `changelog/no-changelog` label added.
- [ ] All relevant GitHub issues are correctly linked.
- [ ] Backports are identified and tagged with Mergifyio.
- [ ] Add to milestone.

(cherry picked from commit 5a6ac70)

Co-authored-by: Alberto Vara <alberto.vara@datadoghq.com>
(cherry picked from commit 523793d)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Brett Langdon <brett.langdon@datadoghq.com>

tox.ini

@@ -72,9 +72,24 @@ isolated_build = true
 
 requires = virtualenv<=20.2.1
 
-[testenv]
+[testenv:gevent_contrib-py{37,38}-gevent{13,14}-sslmodules3-sslmodules]
+# Wheels for gevent segfault pretty easily
+install_command=python -m pip install --no-binary=gevent {opts} {packages}
+usedevelop = true
+
+[testenv:py{37,38}-opentracer_gevent-gevent{13,14}]
 # Wheels for gevent segfault pretty easily
-install_command=python -m pip install --no-binary gevent {opts} {packages}
+install_command=python -m pip install --no-binary=gevent {opts} {packages}
+usedevelop = true
+
+[testenv:py{37,38}-profile-minreqs-gevent]
+# Wheels for gevent segfault pretty easily
+install_command=python -m pip install --no-binary=gevent {opts} {packages}
+usedevelop = true
+
+
+[testenv]
+install_command=python -m pip install {opts} {packages}
 usedevelop = true
 
 setenv =
@@ -86,7 +101,7 @@ extras =
   profile: profiling
 
 deps =
-    cython
+    cython<=0.29.32
     cmake
     ninja
     pytest-cov
@@ -104,8 +119,10 @@ deps =
     profile-!minreqs-gevent: gevent
     py27-profile-minreqs-gevent: gevent==1.1.0
     py{35,36,37,38}-profile-minreqs-gevent: gevent==1.4.0
-    py39-profile-minreqs-gevent: gevent==20.6.0
-    py39-profile-minreqs-gevent: greenlet==0.4.16
+    py39-profile-minreqs-gevent: gevent==20.6.1; sys_platform != 'win32'
+    py39-profile-minreqs-gevent: gevent==21.1.2; sys_platform == 'win32'
+    py39-profile-minreqs-gevent: greenlet==0.4.16; sys_platform != 'win32'
+    py39-profile-minreqs-gevent: greenlet==0.4.17; sys_platform == 'win32'
     py310-profile-minreqs-gevent: gevent==21.8.0
     py310-profile-minreqs-gevent: greenlet==1.1.0
 # backports