drop support for python 2 and add some typing

[fuhrysteve]

Not sure if you're open to dropping python 2 support, but I thought I'd test the waters!

Nothing too exciting, just adding typing on the user-facing zxcvbn() function and in a couple other super easy places, if only to make sure mypy & github actions are actually working.

Please note that this fully breaks backward compatibility with python 3.5 or lower, as it uses features introduced in python 3.6.

[dwolfhub]

Given it's been EOL for a year now it's probably time. I would just want to bump a major version.

[fuhrysteve]

Bumped the major version number to 5 and updated the docs!