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Mathieu, nothing was attached. The penalty's worth only a few if statements, I wouldn't worry too much about it. Besides, a C version is going to be provided as well, right?
Perhaps the following approach might solve the subclasses problem:
regex = dispatch.get(cls)
if not regex:
 classes = datetime, date, time
 cls = next((c for c in classes if issubclass(cls, c)), None)
 if cls is None: 
 raise TypeError
 regex = dispatch[cls]
Perhaps, TypeError is unnecessary there and just propagating StopIteration would do. In that case the if clause would look like:
 classes = datetime, date, time
 cls = next(c for c in classes if issubclass(cls, c))
 regex = dispatch[cls]