Append coverage in CI

Author: nicoddemus

tox.ini

@@ -61,7 +61,9 @@ setenv =
 
     # Configuration to run with coverage similar to CI, e.g.
     # "tox -e py39-coverage".
-    coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run -m
+    # In some cases, we run tox passing multiple environments. Pass --append to ensure runs don't overwrite each
+    # other's coverage data.
+    coverage: _PYTEST_TOX_COVERAGE_RUN=coverage run --append -m
     coverage: _PYTEST_TOX_EXTRA_DEP=coverage-enable-subprocess
     coverage: COVERAGE_FILE={toxinidir}/.coverage
     coverage: COVERAGE_PROCESS_START={toxinidir}/.coveragerc