I took a brief look at the code.  get_dialect is imported from the underlying _csv.c extension module and didn't change between 2.4 and 2.5.  There is a new static function, _call_dialect, though.  It is called from several places, most notably from csv_register_dialect.  The code in 2.4 that it replaces seems much different to me than the code in the body of _call_dialect.  I suspect there was some sort of regression there.

Assigning to Andrew since he's the primary author of the code as well as the author of this particular change.