Upper-pin MyPy to <1.9

MyPy 1.9 has dropped support for Python 3.7.

setup.cfg

@@ -115,7 +115,7 @@ tests =
     flake8-mutable>=1.2.0
     flake8-simplify>=0.14.0
     flake8>=3.0.0
-    mypy>=0.910
+    mypy>=0.910,<1.9
     # https://github.com/pytest-dev/pytest-asyncio/issues/706
     pytest-asyncio>=0.17,!=0.23.*
     pytest-cov>=2.8.0