sdist: include misc/{diff-cache,apply-cache-diff}.py for mypy/test/test_diff_cache.py

[mr-c]

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

[mr-c]

To confirm this PR

$ python3 -m build --sdist
$ tar tf dist/mypy*.tar.gz | grep 'misc/'
mypy-1.20.0+dev.7528f132eee6c3686ff9b2b685e7631af4cf83d4/misc/
mypy-1.20.0+dev.7528f132eee6c3686ff9b2b685e7631af4cf83d4/misc/apply-cache-diff.py
mypy-1.20.0+dev.7528f132eee6c3686ff9b2b685e7631af4cf83d4/misc/diff-cache.py

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Did you check if the scripts work when distributed like this (i.e. can the imports be resolved)?

[mr-c]

Did you check if the scripts work when distributed like this (i.e. can the imports be resolved)?

Yes

Both at build time: https://debusine.debian.net/debian/developers/artifact/3476216/raw/mypy_1.20.0~1dev.7528f132eee6c3686ff9b2b685e7631af4cf83d4-1_amd64-2026-03-24T10:59:04Z.build ( search for -m pytest)
And post-installation: https://debusine.debian.net/debian/developers/artifact/3476260/#L1146-section

[mr-c]

this PR only adds the scripts to the source distribution, not the wheel