typing: add type annotations to the `mercurial.util.filestat` class
It's referenced in the `vfs` classes, so get this out of the way to help there.
The `TypeVar` definition and its usage was copied from the existing `util.pyi`
file.
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true