Remove (direct) use of constantly

[DMRobertson]

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

synapse/synapse/storage/databases/main/events_worker.py

Line 33 in 8a87b44

from constantly import NamedConstant, Names

synapse/synapse/storage/databases/main/events_worker.py

Lines 153 to 160 in 8a87b44

	class EventRedactBehaviour(Names):
	"""
	What to do when retrieving a redacted event from the database.
	"""
	AS_IS = NamedConstant()
	REDACT = NamedConstant()
	BLOCK = NamedConstant()

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

synapse/synapse/handlers/state_deltas.py

Lines 25 to 28 in e24ff8e

	class MatchChange(Enum):
	no_change = auto()
	now_true = auto()
	now_false = auto()

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.

[andrewdoh]

hi, id like to work on this. please check this PR out I made for the outlined above. @DMRobertson
#12624