Bug report

Bug description:

• __main__ handling isn't quite right
• non-pickleable-but-shareable objects fail
• common code with test.support.interpreters should be shared

This has gh-132775 as a prerequisite.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-133485: Use interpreters.Interpreter in InterpreterPoolExecutor #133957
• [3.14] gh-133485: Use interpreters.Interpreter in InterpreterPoolExecutor (gh-133957) #135695