closes #649: remove usage of ABCMeta to prevent performance issues on…

[gonzalocasas]

… IronPython

What type of change is this?

• Bug fix in a backwards-compatible manner.
• New feature in a backwards-compatible manner.
• Breaking change: bug fix or new feature that involve incompatible API changes.
• Other (e.g. doc update, configuration, etc)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I added a line to the CHANGELOG.md file in the Unreleased section under the most fitting heading (e.g. Added, Changed, Removed).
• I ran all tests on my computer and it's all green (i.e. invoke test).
• I ran lint on my computer and there are no errors (i.e. invoke lint).
• I added new functions/classes and made them available on a second-level import, e.g. compas.datastructures.Mesh.
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[beverlylytle]

Don't you think the other classes will be jealous that Base still inherits from ABC?

[tomvanmele]

they were indeed

[beverlylytle]

Should there also be a PR on compas_fab to remove ABCMeta from the backend features?

[gonzalocasas]

Thank you both for the quick review!
@beverlylytle yes, we should do the same for fab.
@brgcode could we please push a new patch release 0.17.2 with this fix?
Thx!

[tomvanmele]

done...