Make reinitialize_command's return type Generic when "command" argument is a Command

[Avasam]

This is a part of adding typeshed's typevars to setuptools, but this change is better handled on distutils' side rather than adding typing-only overloaded overrides in setuptools.

This won't have any effect outside this repo and pypa/setuptools#4704 until pypa/setuptools#4689 , but it also shouldn't have any negative effect by itself.

[jaraco]

Can you explain why these overloads are necessary? In particular, why is the Generic necessary? What is the difference between accepting a Command and a CommandT (bound to Command)?

[Avasam]

Can you explain why these overloads are necessary? In particular, why is the Generic necessary? What is the difference between accepting a Command and a CommandT (bound to Command)?

With generic:

command: Command = ...
install_lib_command: install_lib = ...
reinitialized_command = command.reinitialize_command(install_lib_command)
# Mypy: Revealed type is "distutils.command.install_lib.install_lib"
# Pyright: Type of "reinitialized_command" is "install_lib"
reveal_type(reinitialized_command)

Without generic:

command: Command = ...
install_lib_command: install_lib = ...
reinitialized_command = command.reinitialize_command(install_lib_command)
# Mypy: Revealed type is "distutils.cmd.Command"
# Pyright: Type of "reinitialized_command" is "Command"
reveal_type(reinitialized_command)

The generic allows consumers of this method to not loose on type specificity. Otherwise you'd have to typing.cast back to the original type to use its subclass-specific features. (as can be seen in setuptools)