Fix python 2.7 missing list class

doc/nitpick-exceptions.txt

-py:class sqlalchemy.ext.declarative.api.Base
+# This is not documented as py:exc in Python 2.7 - works in Python 3.x
+py:obj list
+py:class list
 py:exc ValueError
+
+# Other exceptions
+py:class sqlalchemy.ext.declarative.api.Base