Force tox environment to C.UTF-8 locale

Works around https://github.com/ClearcodeHQ/pytest-postgresql/issues/16

tox.ini

@@ -6,6 +6,10 @@ deps =
   .[testing]
   pytest-cov
   pifpaf
+setenv =
+  LC_ALL=C.UTF-8
+  LC_CTYPE=C.UTF-8
+  LANG=C.UTF-8
 commands =
   pifpaf run postgresql -- pytest --cov=swh --cov-branch {posargs}