Sub-collection issue when using ABCMeta

I hit an issue recently when using pymongo inside Abstract Base Class. For example the test.py file raise an exception when executed:

Traceback (most recent call last):

  File "test.py", line 8, in <module>

    a = A()

TypeError: Can't instantiate abstract class A with abstract methods col

It's because ABCMeta try to get attribute __isabstractmethod__ on Collection (https://github.com/python-git/python/blob/master/Lib/abc.py#L80) and Collection return a sub-collection in __getattr__ (https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/collection.py#L130).

A simple fix could be to not return a sub-collection if name begins with "__".

I can write a workaround for this issue but adding "col.__isabstractmethod__ = False" is not very clean.

I hope the issue is clear enough.