As per an internal doc, subscripting a container that does not inherit from typing.Generic can cause errors. In particular, a MutableSequence indexed by enums causes an error of the form
TypeError: Parameters to generic types must be types. This can be triggered, for example, by code of the form

        days_of_week: MutableSequence[dayofweek_pb2.DayOfWeek] = proto.RepeatedField(
            proto.ENUM,
            number=2,
            enum=dayofweek_pb2.DayOfWeek,
        )

which we are seeing in the internal part of our publishing pipeline.

Either of these fixes by itself works:

1. Quoting the type: MutableSequence['dayofweek_pb2.DayOfWeek']
2. Adding from __future__ import annotations at the top of the file. As per an internal reference, this seems to be related to PEP563, which has a status of Accepted as of 2017, so it's not clear why we would need a fix in the first place.

We should decide and implement the right fix.