Use `responses` instead of `httpretty` in tests (#3087)

I have created this PR since
[httpretty](https://github.com/gabrielfalcao/HTTPretty/issues) seems to
be no longer maintained.
Fedora is maintaining httpretty because it is needed in PyGithub.
I think it could be beneficial both for Fedora and PyGithub to move away
from httpretty.

[responses project](https://github.com/getsentry/responses)

Thanks in advance.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>