Exclude tests dir from code coverage (#823)

Author: maxrjones

pyproject.toml

@@ -119,6 +119,9 @@ write_to = "numcodecs/version.py"
 skip = "./.git,fixture"
 ignore-words-list = "ba, compiletime, hist, nd, unparseable"
 
+[tool.coverage.run]
+omit = ["numcodecs/tests/*"]
+
 [tool.coverage.report]
 exclude_lines = [
     "pragma: no cover",