Clean up `typing.rst` docs about `Generic` base classes

[sobolevn]

There are multiple issues right now:

1. Mapping has Mapping(Sized, Collection[KT], Generic[VT_co]) which is not correct. Typeshed's definition: class Mapping(Collection[_KT], Generic[_KT, _VT_co]):. Here we have extra Sized and wrong Generic base class with only one type argument
2. There's also AbstractSet(Sized, Collection[T_co]), which has extra Sized. Collection is always Sized

[kamilturek]

Looks like the issue can be closed.