Marking as *potential* release blocker for 3.10.

Pablo, without this change the newest pip (with [PR 10358]) will not work on Python 3.10 built --with-platlibdir.
This configure option was added in 3.9 for distros that separate `/usr/lib` and `/usr/lib64`, which used downstream patches before.
Things will be broken if:
* this bug is not fixed in 3.10
* AND `pip` switches to _USE_SYSCONFIG in 3.10 (with merged [PR 10358])
* AND distros don't patch again.

So, if this isn't merged in 3.10, either pip or downstreams will need to implement a workaround. (If pip doesn't hold off, downstreams that build  --with-platlibdir will likely carry this exact patch.)
On the other hand, we're very late in the release cycle.

Note that there's a similar bug in bpo-45035

[PR 10358]: https://github.com/pypa/pip/pull/10358