…4038)

Fixes #14031 

It turns out premature optimization is the root of all evil. (It turns
out this costs us less than 1% time on self-check).

Tried to make it in a way such that we can add more rules for platforms
we want to filter out in the future.
Tested it with `python3 misc/upload-pypi.py --dry-run 0.990`
Fixes #14026

#13768 had a bug so that errors were sometimes silenced in files that
were under a directory in `sys.path`. `sys.path` sometimes includes the 
current working directory, resulting in no errors reported at all. Fix it by 
always reporting errors if a file was passed on the command line (unless
*explicitly* silenced).

When using import following errors can still be ignored, which is 
questionable, but this didn't change recently so I'm not addressing it 
here.

Fixes #14042.

Fixes #14048

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

Fixes #14046. Similar to #13733

Best reviewed with hide whitespace.

Fixes #14027 (issue was surfaced by #13509)

improve ast types; revert several "redundant numeric union" changes from #7906
see python/typeshed#9130