Clarify Python 3.8 support (#4784)

.github/workflows/pytest.yml

     strategy:
       matrix:
         python-version: [3.6.x, 3.7.x, 3.8.x]
-        include:
-          - python-version: 3.6.x
-          - python-version: 3.7.x
-          - python-version: 3.8.x
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python

docs/Using-Virtual-Environment.md

 
 ## Python Version Requirement (Required)
 
-This guide has been tested with Python 3.6 and 3.7. Python 3.8 is not supported
-at this time.
+This guide has been tested with Python 3.6 through Python 3.8. Newer versions might not
+have support for the dependent libraries, so are not recommended.
 
 ## Installing Pip (Required)
 

ml-agents-envs/setup.py

         "License :: OSI Approved :: Apache Software License",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
+        "Programming Language :: Python :: 3.8",
     ],
     packages=find_packages(exclude=["*.tests", "*.tests.*", "tests.*", "tests"]),
     zip_safe=False,