I think that you don't use Py_BEGIN_ALLOW_THREADS / 
Py_END_ALLOW_THREADS correctly: the GIL can be released when the 
hashlib lock is acquired (to run hash functions in parallel threads). 
So the macros should be:

#define ENTER_HASHLIB \
   PyThread_acquire_lock(self->lock, 1); \
   Py_BEGIN_ALLOW_THREADS

#define LEAVE_HASHLIB \
   Py_END_ALLOW_THREADS \
   PyThread_release_lock(self->lock);

If I'm right, issue #4738 (zlib) is also affected.