The attached patch adds a simple implementation of time.timegm that
calls calendar.timegm. It includes a short test to show that 
time.timegm(time.gmtime(ts)) == ts for various timestamps.

I implemented a pure C version by pulling in the various functions
needed from datetime, but it was a bit messy and a lot more invasive.

Documentation updates are still needed -- I will do those if there is
support for the patch.