Exclude mypy on PyPy to fix CI

librt (mypyc runtime) doesn't build on PyPy, so exclude mypy from the
test dependency group when running on PyPy.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: RonnyPfannschmidt

setuptools-scm/pyproject.toml

@@ -75,7 +75,7 @@ test = [
   "pytest-xdist",
   "rich",
   "ruff",
-  "mypy",
+  "mypy; implementation_name != 'pypy'",
   'typing-extensions; python_version < "3.11"',
   "griffe",
   "griffe-public-wildcard-imports",