tests: redact TZ environment variable

* Setting the TZ env var within a Python session doesn't work as
  intended.
* We'll add a TZ offset in CI in the near future.

pytest.ini

@@ -34,11 +34,12 @@ testpaths =
     tests/integration/
 env =
     # a weird timezone to check that tests aren't assuming the local timezone
-    TZ=XXX-09:35
+    # Note: this doesn't work properly with pytest-xdist
+    # TZ=XXX-09:35
 doctest_optionflags =
     NORMALIZE_WHITESPACE
     IGNORE_EXCEPTION_DETAIL
     ELLIPSIS
 asyncio_mode = auto
 markers=
-    linkcheck: Test links
+    linkcheck: Test links