CI: requirements.txt: Pin pytest to 6.2.5

This is the version in Ubuntu 22.04; also, `stbt_rig.py setup` installs
this version into the venv it creates.

This fixes a CI failure since pytest 8.0 was released. See #61.

As far as I can tell, this `requirements.txt` is only used by CI (it
isn't mentioned in `setup.py`).

requirements.txt

@@ -1,6 +1,6 @@
 # Dependencies of stbt_rig
 keyring
-pytest
+pytest~=6.2.5
 requests
 tzlocal