Pin mypy version to 0.670 temporarily

mypy version 0.700 includes some stubs for the bleach library, but
they're not correct and cause errors when run on the Tildes codebase.
I've left a comment on the typeshed repo here, so hopefully it will be
resolved before long:
https://github.com/python/typeshed/pull/2709#issuecomment-483120729

I'll pin mypy's version to the previous version of 0.670 for now to
avoid this, but if the stubs aren't fixed for a while I may need to do
it differently.

tildes/requirements-to-freeze.txt

@@ -10,7 +10,7 @@ freezegun
 gunicorn
 html5lib
 ipython
-mypy
+mypy==0.670  # 0.700 includes incorrect bleach stubs
 mypy-extensions
 Pillow
 prometheus-client