Update WebIDL.py
Update WebIDL.py to latest version from mozilla-central.
There were two major changes:
- [Rename legacy extended attributes in webidls](55dac83d9a) [bug 1631581](https://bugzilla.mozilla.org/show_bug.cgi?id=1631581)
- [removal of ReadableStream from WebIDL](https://phabricator.services.mozilla.com/D143074), which was solved with porting reverted removal patch (2801cc82b0)
WebIDL.Parser now has `use_builtin_readable_streams` field by default set to `true` (to use SM readable stream implementation), which could help with #29088.
My try build: https://github.com/sagudev/servo/actions/runs/4216638911
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