Allow stubs to depend on numpy

[AlexWaygood]

Currently, if a stubs package declares a dependency on numpy, the latest version of numpy will be installed in CI when testing that package. However, the CI will then fail, because mypy will detect that numpy is using PEP-570 syntax, and accordingly will emit errors when run with the --python-version=3.7 flag, since PEP 570 is new in Python 3.8.

The solution is to actually run mypy on 3.7 in CI, rather than simply running mypy with the --python-version=3.7 flag. If we run mypy on 3.7, pip's dependency resolver will understand that the latest version of numpy isn't compatible with the version of Python we're using, and will give us an older version of numpy that's compatible with Python 3.7.

A few tweaks to tests/mypy_test.py and tests/utils.py are required in order to make them compatible with Python 3.7. We also can't install the whole of requirements-tests.txt on Python 3.7, since flake8==6.0.0 is not compatible with Python 3.7.

Helps unblock #8974

[hauntsaninja]

Hmm, not sure we should do this. numpy types have evolved a lot in recent versions

[AlexWaygood]

Hmm, not sure we should do this. numpy types have evolved a lot in recent versions

An alternative solution might be to add a requires_python field to the METADATA.toml files. We could use that to declare that e.g. types-tensorflow requires Python 3.8+, and also skip testing those stubs when running mypy with --python-version=3.7.

[hauntsaninja]

Actually, this is fine, since we'll test with new numpy on new Python. (assuming the (lack of) improvements in a numpy don't cause trouble for us)

[hauntsaninja]

Looks good!