Support creating class member variables (not instance members)

[programmerjake]

This can apparently be done by setting PyTypeObject.tp_dict to a dict with the appropriate values before calling PyType_Ready.

See https://stackoverflow.com/questions/35954016/defining-an-inner-class-using-the-python-c-api for a use case. Another use case would be creating enum constants (assuming initialization order can be worked out).

I think a useful way to do that would be to have an overridable static_init function that returns the PyDict used to initialize tp_dict, since that also allows non-const code to create the member variables.

[davidhewitt]

@programmerjake with #905 and #914 is there any more work to be done here? I think we established that the class attributes (as we decided to call them) can't be mutable, so I believe we've covered everything...

[programmerjake]

I think that should be all that's needed, unless we want to add a function (static_init from first comment) that can add members algorithmically, though that would have to be unsafe since it could save a copy of the dict somewhere and try to modify it after initialization finished.

[davidhewitt]

Let's call this done, as nobody else has asked for more on this front since we added this over a year ago. We can open a new issue if there are real-world use cases for the static_init case.