Fix 'Python Import Classes does not import Attributes'.

This commit adds support for parsing class attributes and
includes related test cases. Attribute types are estimated
from the initialisation value.
For supported types see test/import/python/types.py, for
supported visibilities test/import/python/visibility.py

BUG:158739
FIXED-IN:2.20.1 (KDE Applications 16.08.1)