Removes unnecessary packages from setup_requires (#16139)

This change removes unnecessary dependencies from setup_requires:

* docutils are not needed in setup requires and actually
  having them here caused harmful upgrade even if docutils
  are limited to <0.17 elsewhere
* setup_tools should not be needed in setup_requires (by
  the time setup_requires are parsed, they should be already
  installed
* bowler is not needed any more in setup.py (we got rid of it
  when we got rid of backport packages.

Also docutils<0.17 limitation is moved to install_requires
because docutils was already a transitive dependency of
airflow without extras and it could be upgraded even if there
is a limitation in extra.

setup.cfg

@@ -68,10 +68,7 @@ include_package_data = True
 python_requires = ~=3.6
 packages = find:
 setup_requires =
-    bowler
-    docutils
     gitpython
-    setuptools
     wheel
 #####################################################################################################
 # IMPORTANT NOTE!!!!!!!!!!!!!!!
@@ -94,6 +91,12 @@ install_requires =
     cryptography>=0.9.3
     dataclasses;python_version<"3.7"
     dill>=0.2.2, <0.4
+    # Sphinx RTD theme 0.5.2. introduced limitation to docutils to account for some docutils markup
+    # change:
+    #      https://github.com/readthedocs/sphinx_rtd_theme/issues/1112
+    # This limitation can be removed after this issue is closed:
+    #      https://github.com/readthedocs/sphinx_rtd_theme/issues/1115
+    docutils<0.17
     flask>=1.1.0, <2.0
     flask-appbuilder~=3.3
     flask-caching>=1.5.0, <2.0.0

setup.py

@@ -502,12 +502,6 @@ def get_sphinx_theme_version() -> str:
     'bowler',
     'click~=7.1',
     'coverage',
-    # Sphinx RTD theme 0.5.2. introduced limitation to docutils to account for some docutils markup
-    # change:
-    #      https://github.com/readthedocs/sphinx_rtd_theme/issues/1112
-    # This limitation can be removed after this issue is closed:
-    #      https://github.com/readthedocs/sphinx_rtd_theme/issues/1115
-    'docutils<0.17',
     'filelock',
     'flake8>=3.6.0',
     'flake8-colors',