Fix python 3.13 support. (#866)

Python 3.13 dropped module `cgi`, which was made available as a
stand-alone library named `legacy-cgi`. A recent bump in `litellm`
added it as a dependency, breaking their support for python 3.13.

This change adds the stand-alone version of `cgi` library.

Source https://docs.python.org/3/library/cgi.html

Author: blkt