[3.12] gh-123431: Harmonize extension code checks in pickle (GH-123434) (#123460)

This checks are redundant in normal circumstances and can only work if
the extension registry was intentionally broken.

(cherry picked from commit 0c3ea3023878f5ad5ca4680d5510da1fe208cbfa)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>