tox: add {posargs} to pytest invocation

This lets us do e.g.:
```
$ tox -e py39 -- src/pkgcheck/checks/git.py # run just one test
```

Signed-off-by: Sam James <sam@gentoo.org>
Closes: #556
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

Author: thesamesam

tox.ini

@@ -9,7 +9,7 @@ download = true
 extras =
 	test
 commands =
-	pytest
+	pytest {posargs}
 
 # build docs
 [testenv:docs]