Message 95200 - Python tracker

I attached a patch against trunk r76237 which seems to cover this, sans
official tests. I looked around and didn't find a good place for
specifically testing environment variables. I could add test code in
test_warnings if that's the best place.

The patch allows multiple warnings to be passed to the environment
variable as a comma separated string, and works in conjunction with -W.

Let me know what you think.