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)
• [ ] 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 6b855cda9b4cae066c79cc2a10a94e99c2a8df1c and the Trove classifiers for Python 3.7 and 3.8 in commit 215143c9b0712499d25b14cf9bb6c09d66fe4d47.

[slivingston]

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