Exclude test files from coverage report

The coverage report was incorrectly including files from the `tests/`
directory, leading to a confusing and inaccurate summary. This can
happen when `pytest-cov` runs in parallel mode, as it sometimes includes
test files that import the code being measured.

Author: tomschr

.coveragerc

@@ -2,6 +2,8 @@
 concurrency = multiprocessing
 parallel = true
 branch = true
+omit =
+    tests/*
 
 [report]
 precision = 1