Fix mypy errors introduced by GH-1012

[msabramo]

Fix mypy errors introduced by GH-1012

(.venv)
abramowi at Marcs-MacBook-Pro-3 in ~/Code/OpenSource/watchdog (fix-mypy-errors●)
$ tox -e mypy
.pkg: _optional_hooks> python /Users/abramowi/Code/OpenSource/watchdog/.venv/lib/python3.10/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: get_requires_for_build_editable> python /Users/abramowi/Code/OpenSource/watchdog/.venv/lib/python3.10/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
.pkg: build_editable> python /Users/abramowi/Code/OpenSource/watchdog/.venv/lib/python3.10/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
mypy: install_package> python -I -m pip install --force-reinstall --no-deps /Users/abramowi/Code/OpenSource/watchdog/.tox/.tmp/package/11/watchdog-3.0.1-0.editable-cp310-cp310-macosx_12_0_arm64.whl
mypy: commands[0]> mypy
Success: no issues found in 48 source files
.pkg: _exit> python /Users/abramowi/Code/OpenSource/watchdog/.venv/lib/python3.10/site-packages/pyproject_api/_backend.py True setuptools.build_meta __legacy__
  mypy: OK (1.61=setup[1.39]+cmd[0.23] seconds)
  congratulations :) (2.70 seconds)