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`

Author: mr-c

MANIFEST.in

@@ -38,6 +38,8 @@ include test-requirements.in
 include test-requirements.txt
 include mypy_self_check.ini
 prune misc
+include misc/diff-cache.py
+include misc/apply-cache-diff.py
 graft test-data
 graft mypy/test
 include conftest.py