[3.12] gh-110429: Fix race condition in "make regen-all" (GH-110433)

[miss-islington]

"make regen-pegen" now creates a temporary file called "parser.c.new"
instead of "parser.new.c". Previously, if "make clinic" was run in
parallel with "make regen-all", clinic may try but fail to open
"parser.new.c" if the temporay file was removed in the meanwhile.
(cherry picked from commit fb6c4ed)

Co-authored-by: Victor Stinner [email protected]

• Issue: GHA "Check if generated files are up to date" job failed with: No such file or directory: ./Parser/parser.new.c #110429