Switch from travis-ci to GitHub actions, add mypy to build

[fuhrysteve]

Hope you find this PR useful!

This also fixes a few faux-pas items that mypy found

• duplicate imports (fixed by adding __all__ to a module)
• impossible-to-use params on a property decorator

Other things:

• removes unused import deepcopy
• fixes a weird error I was having with the build on python 2.7 using multiple inheritance in the BaseWord class. I simply mimicked the behavior in the inheriting class, which seemed to be a reasonable compromise
• drop support for python 3 versions lower than 3.6 (EOL) while continuing to maintain 2.7 compatibility

Please note that the build won't run for this PR until you merge it (something to do with permissions & this being your repository), however if you'd like to see it passing, you can see it here on my fork: https://github.com/fuhrysteve/plyse/pull/1