support Python 3.7

[slivingston]

The future is upon us. At the time of opening this issue, visiting https://docs.python.org/ results in documentation for Python version 3.7. We should support it:

• add to list of classifiers in setup.py
• add to test matrix on Travis CI (.travis.yml) (EDIT: now done in GitHub Action .github/workflows/main.yml)
• skim the "What's New?" section of the 3.7 announcement, and check for parts of Polytope that might be broken from it;
• if any code changes for compatibility, write regression tests

[slivingston]

also, Python 3.8, which was released after this issue first opened (https://www.python.org/dev/peps/pep-0569/).

[johnyf]

The Trove classifiers for Python 3.7 and 3.8 could be included in polytope == 0.2.2, #58.

[johnyf]

Added Python 3.7 and 3.8 to test environments in commit 6b855cd and the Trove classifiers for Python 3.7 and 3.8 in commit 215143c.

[slivingston]

Sufficiently operational with Python >= 3.8 to close this issue.