gh-132952: Improve Python startup time by ~12%

[mdboom]

This improves the time of a no-site.py startup by 12%, i.e.

python -c pass -S

I'm not strongly of the opinion this should be merged (hence the draft PR), since there are probably all kinds of other places where "lazy importing" could be used to improve startup time that don't improve readability. On the other hand, it's a fairly large regression from 3.13.x that we may want to avoid.

• Issue: #127648 introduced a ~12% performance regression in python_startup_no_site benchmark #132952

[JelleZijlstra]

This only helps the -S startup because site.py imports os.py which also imports _collections_abc.

I think it's pretty defensible to lazy-import here as well as in os.py though; the import is only needed for runtime isinstance() checks on these protocols, and many users will use them without ever needing the isinstance() call.

[sergey-miryanov]

I believe it is related to #118761