Add support for upgrading Python in tool environments (#7605)
This PR adds support for upgrading the build environment of tools with
the addition of a ```--python``` argument to ```uv upgrade```, as
specified in #7471.
Some things to note:
- I added support for individual packages — I didn't think there was a
good reason for ```--python``` to only apply to all packages
- Upgrading with ```--python``` also upgrades the package itself — I
think this is fair as if a user wants to _strictly_ switch the version
of Python being used to build a tool's environment they can use ```uv
install```. This behavior can of course be modified if others don't
agree!
Closes https://github.com/astral-sh/uv/issues/6297.
Closes https://github.com/astral-sh/uv/issues/7471.
