Detect multiple inheritance and add py::multiple_inheritance to it

[virtuald]

Similar to robotpy/robotpy-build#149 , record this information in a .json file and use that to set the flag automatically.

For now, will have to set it manually.

[virtuald]

pybind11 actually will handle this correctly most of the time. The only time that it doesn't handle it is when there's "hidden" MI... eg, the base of your base has MI. I think we might be able to modify pybind11 to detect most of those cases, the only case we'd need something different is if the user ignores a base, then it still needs to be marked as MI... probably. Though, Sendable is ignored everywhere and that's (probably) never caused a problem?