Thinking about this some more, the typing_extensions dependency can be made optional with something like:

if sys.version_info >= (3, 10):
    from typing import TypeGuard

    PathGuard = TypeGuard[bytes | str | Path]
elif typing.TYPE_CHECKING:
    from typing_extensions import TypeGuard

    PathGuard = TypeGuard[bytes | str | Path]
else:
    PathGuard = bool


def is_path(f) -> PathGuard:
    return isinstance(f, (bytes, str, Path))

However, I'm not sure if this is worth doing to avoid the hard dependency, and if so, how the dependency should be declared in pyproject.toml.

Other than for testing, we've managed to avoid Python dependencies so far.

I doubt adding typing_extensions would be a big problem, but if we want to keep zero dependencies, we could move this if/elif/else block into a _typing.py or _compat.py module.

Another option is to leave typing_extensions out, like:

if sys.version_info >= (3, 10):
    from typing import TypeGuard

    PathGuard = TypeGuard[bytes | str | Path]
else:
    PathGuard = bool

And use new Python for type checking; we use 3.12 on the CI. This could also be tidied into an internal module.

After a lot of trial and error, I've managed to make typing_extensions an optional dependency.
As it turns out, my initial suggestion was not functional because mypy doesn't understand a TypeGuard when it is provided as a TypeAlias. The only way I could convince it to accept the type guard in ImageFont was to (in)directly import TypeGuard from either typing or typing_extensions, and provide a fallback class when neither is available (which mypy will never see).

Even if we later add a hard dependency on typing_extensions, I do like the idea of moving the if/else block to _typing.py.