Import from collections.abc when possible

[CasperWA]

Description:

Fixes #236.

Importing ABCs from collections is deprecated and will be removed in Python 3.10.

As a note, I ran pytest and checked the resulting Python warnings. The only ones left are DeprecationWarnings the package raises itself.

Type of change:

• Bug fix.
• New feature.
• Documentation update.

Checklist:

This checklist can be used as a help for the reviewer.

• Is the code easy to read and understand?
• Are comments for humans to read, not computers to disregard?
• Does a new feature has an accompanying new test (in the CI or unit testing schemes)?
• Has the documentation been updated as necessary?
• Does this close the issue?
• Is the change limited to the issue?
• Are errors handled for all outcomes?
• Does the new feature provide new restrictions on dependencies, and if so is this documented?

Comments:

[francescalb]

Perfect.