[Backport rev. 38534 by loewis]

[Backport rev. 38534 by loewis]

[Possibly controversial because it adds a parameter to a method.
 This parameter isn't documented, however, so arguably it's a private
 detail, and the fix is necessary to make GzipFile.flush() behave more
 similarly to regular file object.]

Patch #1110248: SYNC_FLUSH the zlib buffer for GZipFile.flush.
Partially fixes #1110242.