[mod_python] Thread safety

When using a request handler, mod_python calls handler(req) for every
incoming request. I would say it's a safe guess that this implies that
handler can be called concurrently in different threads and anything
done in this function and any functions it calls must be thread safe.
I didn't find any mention of this in the docs (I didn't look too
hard), so I thought I'd verify with the people sure to know.

Thanks,
-Dan