gh-122188: Move magic number to its own file (#122243)

* gh-122188: Move magic number to its own file

* Add versionadded directive

* Do work in C

* Integrate launcher.c

* Make _pyc_magic_number private

* Remove metadata

* Move sys.implementation -> _imp

* Modernize comment

* Move _RAW_MAGIC_NUMBER to the C side as well

* _pyc_magic_number -> pyc_magic_number

* Remove unused import

* Update docs

* Apply suggestions from code review

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>

* Fix typo in tests

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Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>