Use _typeshed.Self where __enter__ returns self

[domdfcoding]

I have made a start on annotating __enter__ methods as def __enter__(self: Self) -> Self: ... where the method just returns the class instance (self)

The current annotations assume that the __enter__ method on subclasses returns an instance of the parent class.

There are more than these, but they are either more complicated or are already annotated as def __enter__(self: _T) -> _T: ...

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This is a step towards #5676, and is related to #5675 and #5602

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

Thank you! We should also use Self for __enter__ methods that are already annotated with a typevar, but obviously that's less urgent.

[JelleZijlstra]

I checked all of these against the implementation and they're indeed just return self.