I try to explain better what is my opinion:
- If you add the attribute by using setAttribute the newlines are kept
and the toxml works well
- If you add the attribute by using the parsestring, passing it an XML
string the newlines are replaced

- Your patch works only if you act on a well-constructed (i.e.with
newlines kept in internal data structures) xml.dom.minidom.Document
object only, so...
- If you try to execute your patched toxml method of a
xml.dom.minidom.Document constructed using parsestring passing it a
string with newline it does not work.

So your patch works only in a specific case: you are trying to fix a
problem in parsestring, acting on its actual parameter.