COMPAT: Import abstract classes from collections.abc
> Changed in version 3.3: Moved Collections Abstract Base Classes to the collections.abc module. For backwards compatibility, they continue to be visible in this module through Python 3.7. Subsequently, they will be removed entirely.

Numpy team has done it. https://github.com/numpy/numpy/pull/10743
```python
if PY33:
    import collections.abc as collections_abc
else:
    import collections as collections_abc
```
A problem is `collections_abc` being too long, `isinstance(something, collections_abc.Iterable)` occupies half line, but `from pandas.compat.collections_abc import Iterable` might pollute namespace.

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BTW, `pandas/compat/__init__.py` seems messy. Since *Much of the code in this module comes from Benjamin Peterson's six library*,  why not require six like many other libraries do?