Allow pickling HTTPStatusError

[camillol]

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Summary

HTTPStatusError's __init__ method has required keyword-only arguments. BaseException has a custom __reduce__ method that can't handle that correctly. As a result, attempting to unpickle a pickled HTTPStatusError fails with:

TypeError: HTTPStatusError.init() missing 2 required keyword-only arguments: 'request' and 'response'

This PR fixes that by defining an appropriate __reduce__ method for HTTPStatusError.

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

Note: I don't think this requires a documentation change, but let me know if so.