bpo-45866: pegen strips directory of "generated from" header (GH-29777) (GH-29792)

"make regen-all" now produces the same output when run from a
directory other than the source tree: when building Python out of the
source tree.

(cherry picked from commit 253b7a0a9fef1d72a4cb87b837885576e68e917c)
5 files changed