Adding support for binding StrEnums

[nicholasjng]

Hey, I have a patch on hand for binding a C++ enum as a StrEnum in Python, and I'm wondering if there's interest in adding it.

Since StrEnum is a Python 3.11 addition, for Python 3.9 and 3.10 the class inherits from (str, Enum) instead, which for my purposes is the same behavior. I opted for this rather than restricting the feature to Python 3.11+, which as far as I know has no precedent anyway.

[wjakob]

It sounds useful but (as usual) depends on how complex the change is.

[nicholasjng]

Done in #1361.