Remove Python 3.5 support

[chrislit]

Python 3.5 support ends on 2020-09-13. At that point, support can be removed, in terms of testing & feature support.

With 3.6, new-style variable type hinting and f-strings can be used.
The random.choices() workaround may be removed.

Other things to consider:
Use the secrets module for RNG outside of testing.

[chrislit]

alphabet: Optional[Union[TCounter[str], Sequence[str], Set[str], int]] = None,
->
alphabet: Optional[Union[TCounter[str], Collection[str], int]] = None,

alphabet: Optional[Union[TCounter[str], Sequence[str], Set[str], int]] = 0,
->
alphabet: Optional[Union[TCounter[str], Collection[str], int]] = 0,

Other cases of Sequence[str], Set[str] in a Union

Essentially, undo 2e4b17c