The entire source tree uses constantly in exactly one file, originally added in #6377.

In other places, we use the stdlib enum module instead. For instance:

I would like us to use the enum approach and ditch constantly, for three reasons:

1. consistency across the source tree.
2. mypy knows what an enum is; it don't know what a NamedConstant is. E.g. reveal_type(EventRedactBehaviour.AS_IS) yields synapse/handlers/message.py:1407: note: Revealed type is "Any".
3. I prefer using a well-known tool from the stdlib rather than the lesser-known constantly.

Task list:

• replace import of constantly with equivalent imports from stdlib enum
• redefine the EventRedactBehaviour type in terms of enum
• remove these lines from mypy.ini.
• run the linter script: in particular, run mypy. Fix any errors that show up.