GH-106160: Fix test_gzip failing under WASI, which does not have zlib. (

#106167)

Fix test_gzip's failure under WASI, which does not have zlib, by using
test.support.import_helper.import_module to import zlib. (gzip
unconditionally imports zlib, so this does not cause any new skips.)

Lib/test/test_gzip.py

@@ -9,14 +9,14 @@
 import struct
 import sys
 import unittest
-import zlib
 from subprocess import PIPE, Popen
 from test.support import import_helper
 from test.support import os_helper
 from test.support import _4G, bigmemtest, requires_subprocess
 from test.support.script_helper import assert_python_ok, assert_python_failure
 
 gzip = import_helper.import_module('gzip')
+zlib = import_helper.import_module('zlib')
 
 data1 = b"""  int length=DEFAULTALLOC, err = Z_OK;
   PyObject *RetVal;