Prevent coverage files from colliding in parallel

Author: dmtucker

tox.ini

@@ -39,7 +39,8 @@ deps =
     packaging ~= 21.3
     pytest-cov ~= 4.1.0
     pytest-randomly ~= 3.4
-
+setenv =
+    COVERAGE_FILE = .coverage.{envname}
 commands = pytest -p no:mypy {posargs:--cov pytest_mypy --cov-branch --cov-fail-under 100 --cov-report term-missing -n auto}
 
 [pytest]