Fix `test_gzip` failure under WASI

[brettcannon]

1858db7 introduced a dependency on zlib in test_gzip (see https://buildbot.python.org/all/#/builders/1046/builds/2315 for the first failure), but zlib is not guaranteed to exist.

/cc @Yhg1s

Linked PRs

• GH-106160: Fix test_gzip failing under WASI, which does not have zlib. #106167
• [3.12] GH-106160: Fix test_gzip failing under WASI, which does not have zlib. (GH-106167) #106170