Adhere to CPython's Stable ABI

1. Identify which bits of PythonCall only depend on the Stable ABI and which bits depend on internals.
2. Work towards only using the Stable ABI.

The issue names no files, tests, or entry points. Begin by inventorying PythonCall's uses of CPython APIs and separating Stable ABI dependencies from internal dependencies; done means the project uses only the Stable ABI where intended.

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