Use mypy type declarations

[mmaslowskicc]

http://mypy.readthedocs.io/en/latest/

This should allow checking if this package code's correctly handles Unicode vs bytestrings types depending on Python version and it should help packages using querystringsafe_base64 do their own type checking.

[fizyk]

@mmaslowskicc are you satisfied with current type declarations? Only running mypy would be left.

[mmaslowskicc]

Yes.