`skeleton`: allow updating non-stable releases for a package

[agriyakhetarpal]

I discovered this today when attempting to SciPy 1.18.0rc1 to 1.18.0rc2 with pyodide skeleton pypi --update

Failed to update scipy: Local version 1.18.0rc1 is newer than PyPI version 1.17.1, cannot update scipy. Please verify in case the version was updated manually and is correct.

We should allow updating to prereleases, alphas, betas, release candidates, and any other kinds of non-stable releases. PyPI exposes this information already. Whoever wishes to pick this should verify the behaviour through a number of edge cases. An inexhaustive list is as follows:

1. A stable release shouldn't update to a prerelease (1.2.3 shouldn't to go 1.2.4rc0)
2. A non-stable release should update only to a non-stable release, except when there is none available. However, if the next stable release is available, we should update to that. (1.2.4.rc0 should update to 1.2.4rc2 and stop, but if a 1.2.5 release is available, update to it).

If there's some better behaviour than this, we should think about it.