setup.py: temporary fix for test dependencies

* Add python_requires metadata. The latest pyroma does check for its
  presence and it makes sense to add it to prevent from installing
  the package on the wrong Python version.
* Set an upper limit for pylint and prospector. A change in pylint broke
  prospector, but we can't upgrade prospector because of an upstream
  bug, see: landscapeio/prospector#545

Change-Id: I2cae1d503639ba35bb0f93515988aff8a207b4f6

setup.py

@@ -44,7 +44,8 @@
         'types-requests',
     ],
     'prospector': [
-        'prospector[with_everything]>=1.3.1',
+        'prospector[with_everything]>=1.3.1,<=1.7.7',  # Temporary upper limit for an upstream regression
+        'pylint<2.15.7',  # Temporary upper limit for a change that breaks prospector that can't be upgraded
         'pytest>=3.10.1',
         'requests-mock>=1.5.2',
     ],
@@ -107,6 +108,7 @@
     name='cumin',
     packages=find_packages(exclude=['*.tests', '*.tests.*']),
     platforms=['GNU/Linux', 'BSD', 'MacOSX'],
+    python_requires='>=3.7',
     setup_requires=setup_requires,
     url='https://github.com/wikimedia/cumin',
     use_scm_version=True,