Fix type hint of `Mocker.__enter__/__exit__`

[jamielennox]

Thank you, checked this compared to the search on the internet. A bit weird that you can't do this in a more supported way, but sure.

[Viicos]

Thank you, checked this compared to the search on the internet. A bit weird that you can't do this in a more supported way, but sure.

I agree it's a bit verbose. As a side note:

• the behavior of the __exit__ method is currently being specified. You can find more info in this PR: https://github.com/python/typing/pull/1718
• I should have said this in the PR description: importing typing_extensions in stub files is fine without explicitly depending on it :+1:

[jamielennox]

Oh, I didn't even register the external import as it all seemed to work. I only recently dropped py2 support, now I should merge all these type definitions back into the regular code base and it will make gaps like this a lot more obvious.

Thanks