'cgi' is deprecated and slated for removal in Python 3.13

[gsnedders]

  /Volumes/gsnedders/projects/wpt/web-platform-tests/tools/wptserve/wptserve/request.py:4: DeprecationWarning: 'cgi' is deprecated and slated for removal in Python 3.13
    import cgi

[jgraham]

The FieldStorage class can typically be replaced with urllib.parse.parse_qsl() for GET and HEAD requests, and the email.message module or multipart for POST and PUT. Most utility functions have replacements.

So it seems like we get to reimplement the bits of FieldStorage that we're using, and we get fully two minor releases between initial deprecation and final removal. Good good.

[nakyong]

when running wpt test on window with python 3.13, it looks having issue bacause of cgi. When it will be fixed?

[MichaelMcDonnell]

It is possible to replace cgi with legacy-cgi. I'm working on supporting Python 3.13 in #49503.

[gsnedders]

It is possible to replace cgi with legacy-cgi.

It's surprising that legacy-cgi requires on Python 3.10, given there's been little of way in changes to cgi/cgitb in many years; I don't see any obvious reason for it to do so.

It is potentially unfortunate that we expose cgi.FieldStorage in our API (and to tests), because it makes it significantly riskier to change.

Given we'd need this vendored either way, maybe we should just copy cgi.py (plus license) into our tree, and effectively start maintaining a fork.

[jgraham]

#49970 is an initial attempt at the plan @gsnedders set out.