Fix style target using sitepackages

We don't need to use sitepackages for that, which also prevents
installing flake8 in there if it's already site installed. We also
don't need future in there.

tox.ini

@@ -18,9 +18,9 @@ allowlist_externals = bash
 deps = future
 
 [testenv:style]
+sitepackages = False
 deps =
     pep8
-    future
     flake8
 commands =
     python ./tools/cpep8.py {posargs}