Tahoe-LAFS depends on removed `cgi` stdlib module

[cypher]

Tahoe-LAFS' allmydata.webish module currently depends on cgi.FieldStorage from the Python standard library in order to "​do terrible things to ensure files are still bytes".

The cgi module, however, was recently (as of Python 3.13) removed as part of the ​PEP-0594 initiative -- of "removing dead batteries from the standard library".

In order to provide compatibility with Python 3.13, then, a suitable cgi replacement module will need to be found and/or the dependent codepaths will need to be updated accordingly. (There may, of course, be other outstanding work/issues that further prevent Python 3.13 compatibility; this is just one such issue).

[meejah]

I think what we should probably do is delete all the weird custom stuff, stop using the "cgi" module entirely and just rely on the Twisted APIs.