coverage: Set patch = subprocess

Subprocess measuring got integrated into coverage.py and removed from
pytest-cov:

https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst#700-2025-09-09
https://coverage.readthedocs.io/en/latest/config.html#run-patch

.coveragerc

@@ -4,6 +4,7 @@ include =
     tests/*
     scripts/*
 branch = true
+patch = subprocess
 omit =
     qutebrowser/__main__.py
     */__init__.py