cursor.fetchmany() should have a default value for size, as per DB-API 2.0 spec

[jpz]

The default value is meant to be cursor.arraysize.

https://www.python.org/dev/peps/pep-0249/#fetchmany

The psycopg implementation here may be of use.

https://github.com/psycopg/psycopg/blob/master/psycopg/psycopg/cursor.py#L607-L626