Fix CI by pinning pytest-timeout down

A new version of pytest-timeout was just released which needs a newer
pytest than what we have installed from the Debian package.

Author: chriskuehl

requirements-dev.txt

@@ -1,3 +1,6 @@
 pre-commit>=0.5.0
 pytest
-pytest-timeout
+# TODO: This pin is to work around an issue where the system pytest is too old.
+# We should fix this by not depending on the system pytest/python packages at
+# some point.
+pytest-timeout<2.0.0