Enable more mypy optional error codes and bump mypy to 0.960

[mkniewallner]

• Added tests for changed code. Not applicable
• Updated documentation for changed code. Not applicable

Similarly to python-poetry/poetry#5665, enable truthy-bool and redudant-expr.
Also update mypy to 0.960.

Note: I also needed to update the tests CI workflow to use PyPy 3.8 instead of PyPy 3.7, which is what we used to have in the past. See comments for the details.

[mkniewallner]

It looks like PyPy 3.7 has troubles installing typed-ast.

It seems to only fail on this pull request because we are updating dependencies here, which invalidates the cache.
Potentially, any PR updating dependencies may end up with failing PyPy tests.

Will try to bump typed-ast to see if it fixes the problem.

[sonarqubecloud]

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[mkniewallner]

I was wondering why tests on PyPy were not failing in the past, and it looks like I wrongly assumed that we used to use PyPy 3.7 when updating the CI workflow in #364.

Looking at an old CI workflow from a PR that also invalidated the cache, we can clearly see that PyPy 3.8 was used when using pypy3, and not PyPy 3.7: https://github.com/python-poetry/poetry-core/runs/6537883827?check_suite_focus=true

Using python3.8 (3.8.10)

Since typed-ast is only required through mypy for Python < 3.8, going back to using PyPy 3.8 make it not try to install the package, which makes the dependencies installation go through.
Sorry for not seeing that in the other PR that updated the CI workflows.

"Required" checks will need to be updated to require pypy-3.8 instead of pypy-3.7.