🧪 Omit third-party coverage @ site-packages

This is a workaround for a coveragepy imperfection where it mistakenly
treats same-namespace third-party modules as same-project ones:
nedbat/coveragepy#1869.

Refs:
* nedbat/coveragepy#876
* nedbat/coveragepy#905
* nedbat/coveragepy@27d8255
* nedbat/coveragepy@5c2f614

.coveragerc

@@ -23,6 +23,9 @@ branch = true
 cover_pylib = false
 # https://coverage.rtfd.io/en/latest/contexts.html#dynamic-contexts
 # dynamic_context = test_function  # conflicts with `pytest-cov` if set here
+# FIXME: drop `omit` post https://github.com/nedbat/coveragepy/issues/1869 fix
+omit =
+  .tox/**
 parallel = true
 plugins =
   covdefaults