Let MutableMapping.pop take a different default type.

[euresti]

This is particularly useful for the idiom d.pop(k, None) to remove an item if it exists.

Fixes #278

[ambv]

Seems a bit arbitrary to remove pop but to leave clear(), etc. We should prune more methods in a follow-up diff.

[ambv]

This is implemented paralleling what get() does.

[JelleZijlstra]

This makes .pop() return a Union, shouldn't we use an overload instead to parallel .get()? With a Union users will have to isinstance check every usage.

[euresti]

I think Union[_T, _T] automatically resolves to _T At least that's what reveal_type show.

from typing import TypeVar, Union

_T = TypeVar('_T')

def pop(var: Union[int, _T]) -> Union[int, _T]:
    return var

reveal_type(pop(7))  # Revealed type is 'builtins.int'
reveal_type(pop('foo')) # Revealed type is 'Union[builtins.int, builtins.str*]'
reveal_type(pop(None)) # Revealed type is 'Union[builtins.int, builtins.None]'

[euresti]

Oh wait. pop has a default value. Let me look.