Instance attributes defined in parent classes can raise InferenceError

[nelfin]

Steps to reproduce

Inferring an attribute on a class instance where the instance attribute is defined in a base class will fail, but
inferring that same attribute via igetattr on an astroid.Instance will succeed.

import astroid
attr = astroid.extract_node("""
class Parent:
    def __init__(self):
        self.data = 123

class Test(Parent):
    pass
t = Test()
t.data
""")
# next(attr.infer())
# raises astroid.exceptions.InferenceError: Inference failed for <Attribute.data l.9 at 0x...>.
instance = next(attr.expr.infer())
print(next(instance.igetattr('data')))
# Const.int(value=123)

Discovered while debugging #904. I couldn't find an exact match for this issue before, but probably pylint-dev/pylint#960 is the same issue.

Current behavior

As above

Expected behavior

Inferring attribute should return Const.int(value=123)

python -c "from astroid import __pkginfo__; print(__pkginfo__.version)" output

2.6.0-dev0