fix: mypy --strict, back to Python 3.9

[mjansche]

• Add py.typed.
• Revert to Union[] for compatibility with Python 3.9.
• Add type annotations to fix mypy warnings.
• Passes mypy --strict with Python 3.8 through Python 3.14.

[NickCrews]

This looks good to me!

on main, this test (which doesn't actually exist, but should) actually does already pass:

def test_bogus_conversion():
    """Tests !z errors"""
    value = "Test"
    assert value
    with pytest.raises(SyntaxError):
        t("hello {value!z}")

This raises SyntaxError: Failed to evaluate expression 'value!z': invalid syntax (<string>, line 1). Which isn't the most useful error message. This PR improves on that.

This PR DOES add a bit more runtime overhead to do this check, but I think it is worth the added complexity and perf hit.

[NickCrews]

As a followup, I think I would prefer if we actually added the infrastructure to keep this passing the type checker. Eg we need to set this up with mypy, or actually I might suggest using ty.

[NickCrews]

@sfermigier can you give this a look? If you give a thumbs up I can rebase on main so it is ready to merge

[sfermigier]

ok.

[NickCrews]

@mjansche our pyproject.toml only says we support python >=3.9. Is there a reason you need to support 3.8? Can you even use this package with 3.8?

[NickCrews]

Also, see this CI run, which runs all the tests and checks on python 3.9. You say Revert to Union[] for compatibility with Python 3.9., but that CI run didn't include that change, and still passed. Am I missing something here, it appears that the code on main still is compatible with 3.9?

[NickCrews]

Ah, it may be because of the --strict flag. We don't use that flag in that linked PR. Once that PR is merged, then I will try adding the --strict flag, getting that to pass on just 3.9-3.14, and then I think we should revisit adding support for 3.8

[mjansche]

@mjansche our pyproject.toml only says we support python >=3.9. Is there a reason you need to support 3.8? Can you even use this package with 3.8?

I don't have a personal reason for using 3.8, but since I was testing my change with different versions, I threw in 3.8 for fun. No need to change the pyrpoject.toml from what it currently is, IMHO.