Types-panda3d

A lot of work has been put into the stubs since the last release, but these are the highlights:
- Stubs for the `direct` package are now included. These are in general less complete than those for `panda3d`, but are still fairly extensive. (This is largely due to the sheer size of `direct`.)
- Multiple inheritance is now taken into account when removing class members implied by inheritance.
- Illusory `__iter__` methods have been added to all classes implementing iteration via `__len__` and `__getitem__`.

Additionally, some noteworthy work has been done outside the package itself:
- The stubs are now checked with `stubtest` and Pyright in addition to mypy.
- The package has been uploaded to PyPI, allowing users to install it using `pip install types-panda3d`.

As always, feedback is welcome and encouraged. (I'd really appreciate some bug reports.)

In addition to general fine-tuning of the stubs to improve their accuracy, this release also adds docstrings to the stub files. While these aren't really related to typing, including them in the stub files allows many IDEs to read and display them without having to inspect the runtime objects dynamically. It's worth noting that, while both the runtime docstrings and those in stubs are generated from comments stored by Interrogate, they aren't always the same. Most obviously, the stub docstrings include comments from different overloads of a function, noting the signature they apply to.

Other notable changes include:

- The minimum required version of Python has been lowered to 3.7 by importing from `typing_extensions` instead of `typing` for newer features.
- `NodePath` is now treaded as a generic type with respect to the type of node it points to.
- In some cases, stubs are skipped for methods and attributes implied by inheritance.
- All enum values exposed to Python as integers are now annotated with `typing.Final`.
- Where possible, method overloads with differing parameter counts are merged by making the final parameter optional.
- With the help of some `type: ignore` comments, the stubs now pass a Mypy check.

While the utility of the package has been greatly improved, it's still incomplete. Close to 100% of the types, methods, and variables available at runtime are covered by the stubs, but their typing isn't always as accurate as it could be. The size of Panda3D means that there are huge chunks that I've never even touched, so I would request that anyone interested in the idea of type-checking Panda3D programs try out the stubs and report any bugs or inaccuracies they come across. The stubs won't be as useful as they can be without insight into how Panda3D is used in practice.

I may have jumped the gun a little with the initial release. This one is a little more refined. Most notably, imports in the stubs are fixed, and some "fake" `__iter__` method stubs have been added to some types that implement iteration via `__getitem__`.

This is the first release of the stub package. Feedback and suggestions are welcome and would be appreciated.
One should be able to simply use `pip install` on the attached `.whl` file to add it some some environment with Panda3D already installed, and a type checker will know what to do.

I initially made a mistake meaning the package didn't actually contain any stubs. That's fixed now.