Support double-digit minor version in python keyword

[cjolowicz]

This fixes an issue where "3.10" is resolved as "python3", as demonstrated by the test case.

Test failure

==================================================================================== FAILURES ====================================================================================
_______________________________________________________ test__resolved_interpreter_numerical_non_windows[3.10-python3.10] ________________________________________________________

sysfind = <MagicMock name='sysfind' id='4339276912'>, make_one = <function make_one.<locals>.factory at 0x102a6e790>, input_ = '3.10', expected = 'python3.10'

    @pytest.mark.parametrize(
        ["input_", "expected"],
        [
            ("3", "python3"),
            ("3.6", "python3.6"),
            ("3.6.2", "python3.6"),
            ("3.10", "python3.10"),
            ("2.7.15", "python2.7"),
        ],
    )
    @mock.patch("nox.virtualenv._SYSTEM", new="Linux")
    @mock.patch.object(py.path.local, "sysfind", return_value=True)
    def test__resolved_interpreter_numerical_non_windows(
        sysfind, make_one, input_, expected
    ):
        venv, _ = make_one(interpreter=input_)

>       assert venv._resolved_interpreter == expected
E       AssertionError: assert 'python3' == 'python3.10'
E         - python3.10
E         ?        ---
E         + python3

.../nox/tests/test_virtualenv.py:344: AssertionError


============================================================================ short test summary info =============================================================================
FAILED tests/test_virtualenv.py::test__resolved_interpreter_numerical_non_windows[3.10-python3.10] - AssertionError: assert 'python3' == 'python3.10'
============================================================== 1 failed, 266 passed, 2 skipped, 1 xfailed in 20.22s ==============================================================

I'm keeping the scope of this PR deliberately restricted to the fix.

Some other things we'd likely want for Python 3.10 support:

• Add 3.10 to Nox's own Nox sessions
• Add trove classifier
• Add Python 3.10 to Appveyor CI
• Add Python 3.10 to Travis CI
• Update Contributor Guide
• Update documentation (config.rst)
• More?

[crwilcox]

LGTM. I thought maybe it makes sense to also make major allow multiple digits, but not sure that is actually a real problem ;)

[cjolowicz]

😀 With an annual release cadence, Python could conceivably switch to Calendar Versioning. But at this stage, it seems to be more loud thinking than a serious proposal.