Fix importing ABC from collections in Python 3.

[tirkarthi]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

Fixes #377 . It also fixes several more instances in addition to the one reported in #377

[coveralls]

Coverage decreased (-0.2%) to 90.866% when pulling 20814d2 on tirkarthi:fix-warnings into 8145740 on IAMconsortium:master.

[danielhuppmann]

thanks @tirkarthi for this fix! I actually started on something similar, see danielhuppmann@c4c8f19 - is there any specific reason for preferring
from six.moves.collections_abc import Iterable
over
from collections.abc import Iterable?

[tirkarthi]

ABC can be imported from collections only in Python. In Python 3 collections.abc is recommended. Using six would ensure it works on both python 2 and 3. I assumed Python 2 is supported by the project. If not then directly using collections.abc is a simple solution. Does the project support Python 2?

[danielhuppmann]

Thanks! I believe that all major packages are dropping support for python 2, and we started using f'{}'-strings for formatting, which are incompatible with versions before 3.6 anyway (afaik)...

[tirkarthi]

Thanks, I will modify the PR

[tirkarthi]

@danielhuppmann I have force pushed my changes to ensure collections.abc is used instead of six.

[danielhuppmann]

Thanks @tirkarthi! Can you still add a line to the release notes? Note that this is the first PR since the last release, so you'll need to insert it between # Next Release and # Release v0.6.0.

[danielhuppmann]

Thanks, resolved the merge conflict for you!

[tirkarthi]

Sorry, I didn't notice and force pushed my change resolving the conflict.

[danielhuppmann]

thanks @tirkarthi for this nice improvement!

[tirkarthi]

Thanks @danielhuppmann for the review and merge :)