sdist: include `misc/{diff-cache,apply-cache-diff}.py` for `mypy/test/test_diff_cache.py` (#21096)
In Debian, we build the `mypy` Debian package from the Python package
source dists published to https://pypi.org/project/mypy/ ; so to support
running the `mypy/test/test_diff_cache.py` tests, include
`misc/{diff-cache,apply-cache-diff}.py` in the `MANIFEST.in`