Pin mypy for regression in v1.12. (#484)

See issue: python/mypy#17960
Fixed in master: python/mypy#17355 after last release v1.3.0.

Author: carltongibson

setup.cfg

@@ -39,7 +39,9 @@ zip_safe = false
 tests =
     pytest
     pytest-asyncio
-    mypy>=0.800
+    # Pending release of https://github.com/python/mypy/pull/17355
+    # Likely in v1.13.1+, when pip can be removed.
+    mypy~=1.11.0
 
 [tool:pytest]
 testpaths = tests