Issue #13959: Re-implement imp.get_suffixes() in Lib/imp.py.

This introduces a new function, imp.extension_suffixes(), which is
currently undocumented. That is forthcoming once issue #14657 is
resolved and how to expose file suffixes is decided.
diff --git a/Python/importlib.h b/Python/importlib.h
index e9f5faf..1681605 100644
--- a/Python/importlib.h
+++ b/Python/importlib.h
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