Incorrect default compresslevel (9) in gzip documentation

[Jaden-Varkey]

Documentation

The default compresslevel for gzip.open(), the GzipFile constructor, and gzip.compress() is COMPRESS_LEVEL_TRADEOFF (6). The signatures and the "Changed in version" notes in Doc/library/gzip.rst already reflect this (it says "the default compression level was reduced to 6, down from 9").
However, a few places still state the default is 9 and should be updated:-

1. the gzip.open() docstring in Lib/gzip.py
2. the GzipFile.init docstring in Lib/gzip.py
3. the GzipFile paragraph in Doc/library/gzip.rst

Linked PRs

• gh-152633: Fix outdated default compresslevel in gzip docs #152634