Allow mypy on PyPy (#111)

pypa/setuptools#4257 shows that mypy now works with PyPy

setup.cfg

@@ -23,9 +23,7 @@ testing =
 	pytest >= 6
 	pytest-checkdocs >= 2.4
 	pytest-cov
-	pytest-mypy; \
-		# workaround for jaraco/skeleton#22
-		python_implementation != "PyPy"
+	pytest-mypy
 	pytest-enabler >= 2.2
 	pytest-ruff >= 0.2.1