Close PyCQA/pylint#2059

In some cases the _proxied object points to another Instance, not to the
Class of an Instance node (usually this happens with enum and with our hardcoded inference support
for this library). Instead of getting a TypeError, we check before if the _proxied matches
what we expect.

Found by #2080

Close PyCQA/pylint#2055

Because they were at the class level, we were considering them BoundMethods, resulting
in a too-many-function-args error when calling these functions.
Close PyCQA/pylint#1965

Backport "Fix submodule import in six.moves" (commit cf564865) to 1.6 release branch

Close PyCQA/pylint#1746

Astroid cannot infer this attribute because of the way
attr builds its classes

Close PyCQA/pylint#1884

Set predicates for transforms which use inference_tips to prevent
a node's _explicit_inference from being overwritten

The augmented assign rhs context path was deleted most likely
due to a recently fixed inference bug where InferenceContext path
attributes were shared between objects.

Recursive functions on the right hand side of the augmented assign would
forget that they were already called, causing an eventual RecursionError
in astroid inference

Now that the InferenceContext clone() method properly copies the
inference path between Contexts, it's fine to remove this hack.

Fixes #437, Fixes #447, Fixes #313, Fixes PyCQA/pylint#1642, Fixes PyCQA/pylint#1805, Fixes PyCQA/pylint#1854, Fixes PyCQA/pylint#1452

pytest redirects stdout so it makes sense that
this test would fail.

Use pytest -s to not skip this test with pytest.

unittest will still use this test.

There was some trouble getting this to work. I needed to use
--pyargs for pytest to work in tox

Allow for test file prefix unittest_ to be changed to standard test_
Helps with updating the astroid tests to standard pytest conventions

Make pytest use proper test directory by default

Copy inference path in inference context upon cloning to prevent diverging inference paths causing uninferable results

Backport of PR #486

Close #473

Close #467

It does not make sense to have these as Attributes, since they aren't inferable per se.

Close #469

This means we can put an Unknown node in RHS of an assignment operations, which will be useful for brain related transforms.

Close PyCQA/pylint#1706