Operations on None should fall back to object

[rwbarton]

rwbarton@morphism:~/mypy$ python3 -m mypy --strict-optional -c 'None == None'
<string>:1: error: Unsupported left operand type for == (None)
rwbarton@morphism:~/mypy$ python3 -m mypy --strict-optional -c 'str(None)'
<string>:1: error: No overload variant of "str" matches argument types [builtins.None]

These should not be errors, as the type of None is a subtype of object.

[ddfisher]

Interestingly, this first of these errors isn't just a strict optional problem!

$ python3 -m mypy -c 'None == None'
<string>:1: error: Unsupported left operand type for == (None)

[ddfisher]

My renaming of this was incorrect -- currently None is a subtype of object, but object isn't in None's MRO.