Run py.test in clean cache directory

And finally remove XDG_CACHE_HOME=... in tox.ini.

pytest.ini

@@ -1,2 +1,7 @@
 [pytest]
 addopts = --assert=plain
+
+# Activate `clean_jedi_cache` fixture for all tests.  This should be
+# fine as long as we are using `clean_jedi_cache` as a session scoped
+# fixture.
+usefixtures = clean_jedi_cache