Since a29456bc, only import lock is used instead of aq_method_lock, dynamic
modules locks and import lock. However, it was creating another deadlock:

when the import lock is held in one thread (for instance when trying to
perform migration to Portal Type as Classes and ZODB Property Sheets), and an
object is loaded from ZODB, a request is sent to ZEO, and this blocks until
another thread (asyncore) gets the ZEO reply and sends it back to the first
thread.

However, if the asyncore thread receives an Exception, it will tries to import
its module and thus create a deadlock as the import lock is still held by the
first thread.