py::end_method_list is defined in autofunction but needed in automodule

[gerrymanoim]

Which causes confusing errors if you only include automodule

[llllllllll]

Ah, we should just include that in automodule.h then. Alternatively, for compilation speed purposes, we could just inline a literal {nullptr} there. That constant is controlled by the CPython API, not us, so we don't need to worry about it changing.