Support for tolerating missing soft dependency

[andy-maier]

The Sphinx package has a "soft" dependency on the "Levenshtein" package: It attempts to import it, but tolerates it when not installed. The corresponding source code is (from https://github.com/sphinx-doc/sphinx/blob/master/sphinx/versioning.py#L22):

try:
    import Levenshtein  # type: ignore[import-not-found]

    IS_SPEEDUP = True
except ImportError:
    IS_SPEEDUP = False

The type: ignore[import-not-found] marker was added in Sphinx 7.3.0. It is used by the "mypy" tool and causes it to tolerate that the package is not installed.

I suggest that pip-check-reqs also introduces support for tolerating a missing soft dependency.