How would this feature be useful?

#1242 introduced the ability to have pipx download standalone python versions, if the requested python version is not available. Currently, there is no way through pipx to delete them again. Having a way for pipx to figure out which interpreters are still in use and delete the ones that aren't would be very useful for housekeeping.

Also see the discussions in the merge review of the feature for more information: #1243 (comment)

Describe the solution you'd like

A new command that checks the interpreters of the installed applications, compares it with the available standalone python versions and deletes the versions that are no longer in use.

Describe alternatives you've considered

Manually going to the PIPX_STANDALONE_PYTHON_CACHEDIR folder and removing python versions, hoping they are not used anymore.