Fix python 2.7 missing list class

doc/nitpick-exceptions.txt

+# 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
+
+# These are real exceptions
 py:class bob.db.base.file.File
 py:class bob.db.base.database.SQLiteDatabase
 py:class sqlalchemy.ext.declarative.api.Base
-py:exc ValueError