[issue21762] update the import machinery to only use __spec__

[Brett Cannon]

Brett Cannon <brett at python.org> added the comment:

It turns out that the import system itself doesn't use `__loader__` (it does set it), but various parts of the stdlib do use `__loader__`. bpo-42133 updates a bunch of stdlib modules to use `__spec__.loader` as a fallback. bpo-42132 tracks the fact that there's C code which isn't setting or using `__spec__` (let alone `__spec__.loader`).

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue21762>
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