MAINT: import Iterable from collections.abc (Python 3.9 compatibility)

[grlee77]

Description

This PR fixes a DeprecationWarning in the test suite related to an outdated import of Iterable. I think this will become an error starting with Python 3.9.

This warning was listed at #4454 (comment). I can take a look at some of the other ones there as well during the sprint.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[alexdesiqueira]

@grlee77 thank you Greg!