[3.12] GH-105808: Fix a regression introduced in GH-101251 (GH-105910) (#105920)

GH-105808: Fix a regression introduced in GH-101251 (GH-105910)

Fix a regression introduced in pythonGH-101251, causing GzipFile.flush() to
not flush the compressor (nor pass along the zip_mode argument).
(cherry picked from commit 1858db7cbdbf41aa600c954c15224307bf81a258)

Co-authored-by: T. Wouters <thomas@python.org>