Issue #9260: A finer-grained import lock.

Most of the import sequence now uses per-module locks rather than the
global import lock, eliminating well-known issues with threads and imports.
diff --git a/Python/importlib.h b/Python/importlib.h
index cf5619a..493cd1a 100644
--- a/Python/importlib.h
+++ b/Python/importlib.h
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