Add check if python_version was parsed as float in pyproject.toml

[Marcel-ICMC]

Fixes #12108

Passing python_version = 3.10 in a toml file without quotes gives this error message

pyproject.toml: [mypy]: python_version: Python 3.1 is not supported (must be 3.4 or higher)

This happens because the argument is parsed as a float and is converted into a string in mypy code, turning 3.10 into '3.1'. For other version like 3.8 this isn't a problem since it doesn't end in a zero.

I decided to leave python versions that don't end in a zero without error message, since it doesn't seem to have any problems regarding those cases. But for 3.10 a more clear message should leave fewer people confused about this specific issue.

Test Plan

Added a test for the case of python_version = 3.10 with the expected message at cmdline.test.

[hauntsaninja]

Thanks for the PR! Can we move this check inside parse_version? We could also maybe give a more helpful message. E.g. something like

if minor < defaults.PYTHON3_VERSION_MIN[1]:
    msg = "..."
    if isinstance(s, float):
        msg += "You may need to put quotes around your version"

[hauntsaninja]

Thank you!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉