Remove version constraint of pytest

Previously, in f801b69, we had to limit pytest to version <7 because our
tests were using pytest-forked to run D-Bus unit tests and pytest was
crashing on that.

We have since changed the way we test D-Bus by not starting a server for
each test case, and aren't constrained by the pytest regression anymore.

test-requirements.txt

@@ -9,7 +9,7 @@
 # please update the version there in case it is bumped here
 black==23.3.0
 flake8
-pytest<7
+pytest
 pytest-randomly
 pytest-timeout
 pytest-cov