ENH: Add `dict.get(key, None)` → `dict.get(key)` as SIM910

[kyoto7250]

close #171

This PR implements the rules of SIM910. This rule detects dict.get(key, None).

ref: https://docs.python.org/3/library/stdtypes.html?highlight=dict#dict.get

[kyoto7250]

We should use BOOL_CONST_TYPES for Python3.7 and earlier compatibility.

[MartinThoma]

Thank you :hugs: I'll release flake8-simplify==0.20.0 with that change today