gh-124397: Add itertools.serialize (name tbd)

[eendebakpt]

Add itertools.serialize to make a non-threadsafe iterator thread-safe.

• Bikeshedding on the naming (thread at discuss.python.org)
• Update documentation
• Add method to make generator function thread-safe (probably a followup PR)

• Issue: Strategy for Iterators in Free Threading #124397

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📚 Documentation preview 📚: https://cpython-previews--133272.org.readthedocs.build/

[rhettinger]

Do you think this belongs in itertools or in threading? I could see a case for either one.

[eendebakpt]

Do you think this belongs in itertools or in threading? I could see a case for either one.

I see your point. Serhey also mentioned threading as a better option on discourse. I think I would prefer moving to the threading module, but also add link to the new method in the itertools documentation.

[eendebakpt]

Closing in favor of #133908