Replace ctypes.DllGetClassObject and remove DllCanUnloadNow

As far as I can tell, these functions are hooks: third-party code is meant to replace them.

Their implementation in ctypes (i.e. their default behaviour) is to import and call the same-named functions from a third-party library, comtypes.server.inprocserver. This is not good. comtypes should instead register their hook on import.

Here's a possible plan to make the API boundary better without breaking users.

DllCanUnloadNow

While the Python interpreter is running, it is not safe to unload the shared library that contains _ctypes. Therefore:

• The C function DllCanUnloadNow exported from _ctypes should be changed to always return S_FALSE. We should change that now, without a deprecation period. (Note that the comtypes hook already does this.)
• We should stop importing and calling comtypes.server.inprocserver. I'm not sure about the necessary deprecation period, but I think that it should be a non-breaking change and can also be done immediately. Or is someone relying on it for side effects? O_o
• Setting and getting the hook should be deprecated. In about Python 3.18 we should stop calling it, and remove it.

DllGetClassObject

This one, on the other hand, sounds like a useful hook. It also looks like an inprocess COM server need a special build so it's not useful to allow multiple hooks -- replacing a global one is enough. Is that so?
If yes:

• ctypes.DllGetClassObject (the default implementation) should raise a DeprecationWarning. In about Python 3.18, it should be changed to do nothing, just, return CLASS_E_CLASSNOTAVAILABLE.
• comtypes should be changed: on import, it should replace ctypes.DllGetClassObject with its own hook.

This should ensure that old versions of comtypes still work as before (until after the deprecation period).

Does that sound reasonable?
cc @junkmd

Linked PRs

• gh-127766

Start by reviewing the linked pull request gh-127766 and the issue's proposed changes to ctypes.DllGetClassObject and DllCanUnloadNow. Trace the existing ctypes hooks and the comtypes.server.inprocserver integration described here; done means the API and deprecation behavior are agreed, implemented without unintended comtypes breakage, and covered by the relevant tests.