fix(deps): allow webtest to work on python 3.13

[lbillinghamwrk]

Should fix issue #1721.

webtest is used to validate incoming webhooks in the production Webserver().

Via webtest and webob, we have a transitive dependency on cgi. As of python 3.13 cgi has been dropped from stdlib.

The recommended stopgap is to add an explicit dependency on legacy-cgi, long term webob should be replaced with WSGI or ASGI

[lbillinghamwrk]

I'm not 100% sure that setup.py is compatible with PEP 508 environment markers. Pending a full test-suite workflow approval, I've tested that python -m pip install . works in both python 3.9 and 3.13. Only the latter, as expected, has legacy-cgi present.