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Here's a checkpoint of current progress.  The patch applies
cleanly and even compiles.  It works most of the time, but
it causes a bunch of test failures.  I haven't had time to
debug the errors, two likely errors are incorrect
propagation of errors from across the release lock boundary.
 (The error checking goes on inside so that clearerr() can
be called while the file lock is held but
PyErr_SetFromErrno() can be called while the GIL is held.) 
The other source of errors is typos -- like calling LOCK
when you mean UNLOCK.