Travis CI: Upgrade to Python 3.7 and add flake8 tests

[cclauss]

• Add Python 3.7 to Travis CI and tox in alignment with travis-ci/travis-ci#9069
• Add flake8 tests to look for Python syntax errors and undefined names

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

[webknjaz]

Still, some compat issues should be fixed.