Incorrect default compresslevel (9) in gzip documentation

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-152634

閱讀 Lib/gzip.py 和 Doc/library/gzip.rst，對照現有的簽名與版本說明檢查列出的三個 docstring 和段落位置。完成的標準是，這些位置中所有剩餘的敘述都一致地將預設壓縮等級描述為 6，而不是 9。