Add test coverage for _multidict_base.py

[a5r0n]

NotImplemented and Iterable but not Set for _viewbaseset_{and, or, sub, xor}

Partially addresses #921.

[a5r0n]

@webknjaz if I'm not wrong using and, or, -, ^ will give errors from pytest, but I will check that once again

[webknjaz]

@a5r0n thanks for starting this effort! Those operators shouldn't cause problems in pytest. However, linters (like flake8) will be unhappy about unassigned expressions. To address that, use # noqa comments with specific error codes and a comment text explaining why that linting ignore is necessary.

[webknjaz]

Oh, and the current CI failures don't seem to be related to your patch. They'll require addressing separately. I'll have to take a look myself. You'll need to rebase your PR branch at some point, once that's fixed.

P.S. Here's a recommendation for the future PRs on GitHub: https://hynek.me/articles/pull-requests-branch/.

[webknjaz]

@a5r0n I clicked "rebase" so this PR picks up the CI fixes.