This change makes mypy complain if the constants are ever reassigned,
and, more usefully, makes mypy type them as `Literal`s instead of `str`s,
allowing code of the following form to pass mypy:
```py
def do_something(membership: Literal["join", "leave"], ...): ...

do_something(Membership.JOIN, ...)
```

This change makes mypy type the constants as `Literal`s instead of
`str`s, allowing code of the following form to pass mypy:
```py
def do_something(
    membership: Literal[Membership.JOIN, Membership.LEAVE], ...
):
    ...

do_something(Membership.JOIN, ...)
```

This reverts commit b43d085.