cgitb is going away and gives warnings when importing, and that make tests
fail:
  $TESTTMP/hgweb.cgi:5: DeprecationWarning: 'cgitb' is deprecated and slated for removal in Python 3.13

The lack of a "nice" high level error handler is not a huge problem, neither
for users (where it is disabled anyway) or for tests (where we don't use a
browser and the plain tracebacks often are more readable). It is inevitable
that it is going away, and there is no obvious alternative. Remove it and move
on.