Handle `OpenSSL.SSL.WantReadError` and `WantWriteError` in pyopenssl adapter

[vashek]

❓ What kind of change does this PR introduce?

• [x] 🐞 bug fix
• [ ] 🐣 feature
• [ ] 📋 docs update
• [ ] 📋 tests/coverage improvement
• [ ] 📋 refactoring
• [ ] 💥 other

📋 What is the related issue number (starting with #)

Fixes #245

❓ What is the current behavior? (You can also link to an open issue here)

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Checklist:

• [ ] I think the code is well written
• [ ] I wrote good commit messages
• [ ] I have squashed related commits together after the changes have been approved
• [ ] Unit tests for the changes exist
• [ ] Integration tests for the changes exist (if applicable)
• [ ] I used the same coding conventions as the rest of the project
• [ ] The new code doesn't generate linter offenses
• [ ] Documentation reflects the changes
• [ ] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

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This change is 

[webknjaz]

@vashek have you considered fixing pyopenssl itself? https://github.com/pyca/pyopenssl/issues/176#issuecomment-720471766

[webknjaz]

Hm... it looks like sendall was used as write() + flush() at some point but then got replaced with write: https://github.com/cherrypy/cheroot/commit/60cd709551d564551d91a5b170c311bb23ad2ac6 / https://github.com/cherrypy/cheroot/commit/a0b1d018c912aa21473bac3f99d46018faed5563.

[webknjaz]

Some jobs are failing because of a deprecation warning in cryptography. I've fixed that on master. To get the changes, rebase this PR branch.

[webknjaz]

Do you think this can be tested somehow?

[webknjaz]

@vashek so what do you think about tests?

[webknjaz]

The test will probably need to make the sending and/or receiving socket full like I tried here: https://github.com/pyca/pyopenssl/pull/955/files#diff-5fbedfbbf8f0780aeee680927973f302330dc10fe915c7e7deecf4b0556c492cR3157

[vashek]

Sorry for letting this rot. Quite honestly, I'd appreciate if someone could help with the tests - or accept it without them.

[vashek]

Any chance of accepting this?

[webknjaz]

@vashek I suppose it's hard to write tests for this which is why I'll probably accept it without tests. But https://github.com/cherrypy/cheroot/issues/245#issuecomment-1465341867 seems to imply that it may be causing problems for retries. Do you have any thoughts on that?