[3.7] bpo-39389: gzip: fix compression level metadata (GH-18077) (GH-18101)

* bpo-39389: gzip: fix compression level metadata (GH-18077)

As described in RFC 1952, section 2.3.1, the XFL (eXtra FLags) byte of a
gzip member header should indicate whether the DEFLATE algorithm was
tuned for speed or compression ratio. Prior to this patch, archives
emitted by the `gzip` module always indicated maximum compression.
(cherry picked from commit eab3b3f1c60afecfb4db3c3619109684cb04bd60)

Co-authored-by: William Chargin <wchargin@gmail.com>

• Lib/gzip.py[diff]
• Lib/test/test_gzip.py[diff]
• Misc/NEWS.d/next/Library/2020-01-20-00-56-01.bpo-39389.fEirIS.rst[Added - diff]