Pin mypy version in tox
Currently pinning to 0.910 due to a type issue with __version_info__.
@@ -29,7 +29,7 @@ commands = pre-commit run --all-files
# `webargs` and `marshmallow` both installed is a valuable safeguard against
# issues in which `mypy` running on every file standalone won't catch things
[testenv:mypy]
-deps = mypy
+deps = mypy==0.910
extras = frameworks
commands = mypy src/
