Fix outdated default compresslevel in gzip docs

The default compression level for gzip.open(), the GzipFile constructor
and gzip.compress() was lowered from 9 to 6 (it now uses
_COMPRESS_LEVEL_TRADEOFF), and the signatures and versionchanged notes
were updated accordingly. A couple of docstrings and one paragraph in
the library docs still said the default was 9, which was misleading.
Update them to match the actual default of 6.

Author: Jaden-Varkey

Doc/library/gzip.rst

@@ -112,7 +112,7 @@ The module defines the following items:
    The *compresslevel* argument is an integer from ``0`` to ``9`` controlling
    the level of compression; ``1`` is fastest and produces the least
    compression, and ``9`` is slowest and produces the most compression. ``0``
-   is no compression. The default is ``9``.
+   is no compression. The default is ``6``.
 
    The optional *mtime* argument is the timestamp requested by gzip. The time
    is in Unix format, i.e., seconds since 00:00:00 UTC, January 1, 1970. Set

Lib/gzip.py

@@ -39,7 +39,7 @@ def open(filename, mode="rb", compresslevel=_COMPRESS_LEVEL_TRADEOFF,
 
     The mode argument can be "r", "rb", "w", "wb", "x", "xb", "a" or "ab"
     for binary mode, or "rt", "wt", "xt" or "at" for text mode.  The default
-    mode is "rb", and the default compresslevel is 9.
+    mode is "rb", and the default compresslevel is 6.
 
     For binary mode, this function is equivalent to the GzipFile
     constructor: GzipFile(filename, mode, compresslevel).  In this case,
@@ -186,7 +186,7 @@ def __init__(self, filename=None, mode=None,
         The compresslevel argument is an integer from 0 to 9 controlling
         the level of compression; 1 is fastest and produces the least
         compression, and 9 is slowest and produces the most compression.
-        0 is no compression at all. The default is 9.
+        0 is no compression at all. The default is 6.
 
         The optional mtime argument is the timestamp requested by gzip.
         The time is in Unix format, i.e., seconds since 00:00:00 UTC,