Allow running custom posargs without code coverage

Author: dmtucker

tox.ini

@@ -137,7 +137,7 @@ deps =
 
     pexpect ~= 4.8.0
 
-commands = py.test -p no:mypy --cov pytest_mypy --cov-fail-under 100 --cov-report term-missing {posargs:-n auto}
+commands = py.test -p no:mypy {posargs:--cov pytest_mypy --cov-fail-under 100 --cov-report term-missing -n auto}
 
 [pytest]
 testpaths = tests