Deal with marshmallow-enum dead end

[obscurerichard]

We rely on Marshmallow for object serialization.

However, not all parts of this ecosystem are well-maintained.

This is a nasty little one because we rely on this for mapping incoming Strava API JSON to our ORM objects. The marshmallow-enum feature relies on a bug fix Lyft made to this now-archived library. See justanr/marshmallow_enum#34 and https://github.com/lyft/marshmallow_enum/tree/support-for-marshamallow-3 . We were able to get to Marshmallow 3 but this dependency is kind of difficult.

Consider switching to another library or reworking our code so we don't depend on the enum part anymore.