test: import pytest in test_stderr_timestamp (marker needs it)

The spawns_gateway_lookalike marker was added to a module that never imported
pytest; collection failed with NameError in CI.

tests/hermes_cli/test_stderr_timestamp.py

@@ -3,6 +3,8 @@
 import re
 import sys
 
+import pytest
+
 from gateway.restart import EXTERNAL_GATEWAY_SUPERVISOR_ENV
 from hermes_cli import stderr_timestamp