Fix base getattro

[stephan-hof]

Explanation why this pull request is needed can be found here: #7 (comment)
For further discussion lets use this pull request.

I split the change to several commits, so its more obvious what belongs to the python3 port or to our extension.

I also wrote a unittest which worked with the C-Extension of ExtenionClass 4.1.2
This test fails now with pypy. So it looks like the python only version suffers the same problem.

[stephan-hof]

I added a commit which fixes the python version too.
The python version of Base_getattro simulates object.__getattribute, which makes it possible to inject our conditional call to of_get on an attribute coming from the instance dict.

[mauritsvanrees]

I can confirm that this fixes failures in the Zope2 tests when using an ExtensionClass (and AccessControl) checkout.

[hannosch]

I've looked at this and it looks good to me. The customized _PyObject_GenericGetAttrWithDict hasn't changed up until Python 3.6, so we should be good there. I've opened #13 for Python 3.7 compatibility in the future.

[seanupton]

FYI related to this change: in upgrading an existing Plone add-on to work with Plone 5.1b4 (which incorporates ExtensionClass 4.3.0), I had to change wrapping level used for ComputedAttribute (via a decorator on a class method) from level=1 (in ExtentionClass <= 4.2.0) to level=0 (in >= 4.2.1).