Remove unnecessary mock dependency (#1572)

Presumably left over from Python 2, since Python 3 added mock to the standard library.

Author: adamchainz

tox.ini

@@ -9,7 +9,6 @@ envlist =
 deps =
     dj32: django~=3.2.9
     dj40: django~=4.0.0
-    sqlite: mock
     postgresql: psycopg2-binary
     postgis: psycopg2-binary
     mysql: mysqlclient