py3: use setattr() to assign new class attribute

The old method produces error 'object does not supports item assignment'.
So setattr() is used to assign a new class attribute via __dict__ .

mercurial/mail.py

@@ -41,7 +41,7 b' def _unifiedheaderinit(self, *args, **kw'
     kw['continuation_ws'] = ' '
     _oldheaderinit(self, *args, **kw)
 
-email.Header.Header.__dict__['__init__'] = _unifiedheaderinit
+setattr(email.header.Header, '__init__', _unifiedheaderinit)
 
 class STARTTLS(smtplib.SMTP):
     '''Derived class to verify the peer certificate for STARTTLS.