pydocstyle is not needed to run tests.
See also https://github.com/Bachmann1234/diff_cover/issues/424

--- diff_cover-9.7.1/pyproject.toml.orig
+++ diff_cover-9.7.1/pyproject.toml
@@ -55,7 +55,6 @@
 pycodestyle = ">=2.9.1"
 pyflakes = "^3.3.2"
 pylint = "^3.3.4"
-pydocstyle = "^6.1.1"
 black = "^25.1.0"
 isort = "^6.0.1"
 ruff = ">=0.11.10,<0.13.0"