Add uts46 and transitional to idna.encode/decode to support legacy emoji and idna domains

[john-parton]

Scope and purpose

Fixes #10078

Should fix scrapy/scrapy#4330 (comment) once Scrapy uses version of Twisted with patch

Still need tests.

You can see some discussion on exactly what this does here: kjd/idna#18
And also the IDNA docs: https://github.com/kjd/idna#compatibility-mapping-uts-46

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I know you're not ready for review here yet, but I'm looking forward to this: thank you for the contribution!

[john-parton]

I know you're not ready for review here yet, but I'm looking forward to this: thank you for the contribution!

I don't think I'm going to be able to add a test, docs, or anything supplementary, unfortunately.