PEP 705: Mark as Final

[hugovk]

• Final implementation has been merged (including tests and docs)
• PEP matches the final implementation
• Any substantial changes since the accepted version approved by the SC/PEP delegate
• Pull request title in appropriate format (PEP 123: Mark Final)
• Status changed to Final (and Python-Version is correct)
• Canonical docs/spec linked with a canonical-doc directive
  (or canonical-pypa-spec for packaging PEPs,
  or canonical-typing-spec for typing PEPs)

Helps #3781, cc @alicederyn.

The spec link goes to:

https://typing.readthedocs.io/en/latest/spec/typeddict.html#typing-readonly-type-qualifier

Or do we want to link to the header just above it? We'll need to add an extra ref to the spec, if so.

https://typing.readthedocs.io/en/latest/spec/typeddict.html#read-only-items

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