pytest.ini: drop required coverage to 70%

We haven't invested too much in trying to unit test our Python code. It's
mostly glue code stuff that's better tested with integration tests like
we do in CI and pipelines.

Let's only lower the threshold here for now but in the future we should
probably consider just dropping a threshold entirely.

Prompted by #3618 which triggered this.

pytest.ini

@@ -1,2 +1,2 @@
 [pytest]
-addopts = --cov=cosalib.cli --cov=cosalib.meta --cov=cosalib.cmdlib --cov-report term --cov-fail-under=75
+addopts = --cov=cosalib.cli --cov=cosalib.meta --cov=cosalib.cmdlib --cov-report term --cov-fail-under=70