Add specific Python 3 versions supported to Trove classifiers

[johnthagen]

It appears the CI build only tests Python 3.7 and 3.8:

tinyaes-py/.github/workflows/pythonpackage.yml

Line 10 in bf1b6a3

python-version: [2.7, 3.7, 3.8]

Since Python 3.6 is supported until December 2021, could Python 3.6 support be added and wheels published for macOS and Windows?

The setup.py trove classifiers could then be updated to indicate which Python 3 versions are supported:

tinyaes-py/setup.py

Line 62 in bf1b6a3

"Programming Language :: Python :: 3",

 "Programming Language :: Python :: 3.6",
 "Programming Language :: Python :: 3.7", 
 "Programming Language :: Python :: 3.8",

PyInstaller will be using tinyaes on it's next release, so I expect an increase in user base at that time.