Remove support for the `pkg_resources` metadata backend.

[sbidoul]

Using pkg_resources (part of setuptools) as library, is deprecated.

On Python 3.11+, pip does not use it by default, although it can still be enabled using an undocumented environment variable or a global flag to be set by redistributors. On Python 3.14+, pip does not support it.

We plan to remove support for using pkg_resources to detect installed distribution when pip stops supporting Python 3.10, not earlier than 26.3 (October 2026).

If you reach this issue from the deprecation warning, it is likely that the undocumented environment variable is set. To silence the warning, please unset it, or contact your systems administrator.