CLN: Import abc from collections.abc in prep for py3.8
 #23750, and various issues on supporting upcoming python 3.8.

Azure (multiple):
```
[warning] Using or importing the ABCs from 'collections' instead 
of from 'collections.abc' is deprecated, and in 3.8 it will stop working
```

From https://docs.python.org/3/library/collections.html?highlight=collections#module-collections:
```
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.
```

You're right, I saw the test warning looked it up and completely misread the release notes. 
It's `from collections import ABCfoo` which will break with 3.8, and I can't find any. Sorry for the noise.

 No problem, the warnings are not always very clear in where they are coming from ..