Use values of READ and WRITE from the gzip module

These changed from integers to mode strings in Python 3.13, and we need
to remain consistent with the standard library.

Fixes #44.

Reference:
https://docs.python.org/3.13/library/gzip.html#gzip.GzipFile.mode

Author: musicinmybrain

src/zlib_ng/gzip_ng.py

@@ -41,7 +41,7 @@
 READ_BUFFER_SIZE = 512 * 1024
 
 FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16
-READ, WRITE = 1, 2
+READ, WRITE = gzip.READ, gzip.WRITE
 
 BadGzipFile = gzip.BadGzipFile  # type: ignore