Pin mypy to '<0.990' due to realpython/pytest-mypy#141

Author: jaraco

setup.cfg

@@ -40,6 +40,8 @@ testing =
 	pytest-mypy >= 0.9.1; \
 		# workaround for jaraco/skeleton#22
 		python_implementation != "PyPy"
+	# workaround for realpython/pytest-mypy#141
+	mypy < 0.990
 	pytest-enabler >= 1.3
 
 	# local