Pin pytest to 7.4.3

Something changed in 7.4.4 causes errors in test collection
with multiple conftest.py.  Test run output:
https://github.com/nocarryr/tslumd/actions/runs/7388259172/job/20098822940

Possibly caused by pytest-dev/pytest#11747 / d06c05bd ?

requirements-dev.txt

@@ -1,4 +1,4 @@
-pytest
+pytest==7.4.3
 pytest-asyncio
 pytest-cov
 pytest-doctestplus