Warn on unsafe value of 'sep=' in Signer

Issues a warning if the the separator passed to `django.core.signing.Signer` is an invalid value.

See also: https://code.djangoproject.com/ticket/22804
probably want something like: `sorted(_SEP_UNSAFE)` so the order isn't random. Might be clearer to output something like "0-9A-Za-z-_=" though.