Fix __getitem__() for various index types

[Dr-Irv]

• Closes PeriodIndex element start_time and end_time attributes raise error #536
• Tests added:
  • test_indexes.py:test_getitem()

By making __getitem__() a mixin, it uncovered some odd hierarchy in the index types, where we had redundant declarations, and some incorrect detection of some invalid subtraction operations. So the changes here make the original tests pass, and then we can test __getitem__() on various index types.

Was able to get rid of a number of #type: ignore statements as a result.

Either @bashtage or @twoertwein can review/approve.

[twoertwein]

I assume only Index inherits from this. Can you use _IndexGetitemMixinT for self/return value?

[twoertwein]

Could probably also start using typing.Self

[Dr-Irv]

I assume only Index inherits from this. Can you use _IndexGetitemMixinT for self/return value?

Your assumption isn't correct. DatetimeIndex, PeriodIndex and TimedeltaIndex also inherit, and then are specifying the return type of __getitem__(x: int) via the Generic parameter.

Could probably also start using typing.Self

I tried creating a TypeVar as _IndexGetitemMixinT and it did work. Tried typing.Self, and it doesn't work. I think the type of self in this case has to be "bound" to the return type.

These changes are in commit 0db8a00

[twoertwein]

Thank you @Dr-Irv !