bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726)

bpo-37579: Improve equality behavior for pure Python datetime and time (GH-14726)

Returns NotImplemented for timedelta and time in __eq__ for different types in Python implementation, which matches the C implementation.

This also adds tests to enforce that these objects will fall back to the right hand side's __eq__ and/or __ne__ implementation.

bpo-37579