Add support for django's enum types

[Mojken]

Seems like a reasonable idea, doesn't take much to support it. I don't know much about this project, is it meant to be used exclusively with django? If not, this makes django a dependency, and thus is a breaking change. A separate django module might be more interesting in that case. If it is, then this closes #3.

[antonagestam]

@Mojken This is not meant to be used exclusively with Django, so the dependency/support needs to be made optional.

[antonagestam]

Note that TextChoices can also be instantiated without explicit labels. This is valid:

class D(TextChoices):
    a = "a"