Fix deprecated import for Iterable

[cgoldberg]

This PR fixes the way Iterable is imported. With this change, it will import it from collections.abc rather than collections (which emitted a deprecation warning).

Explanation:
Importing abstract base classes from collections instead of from collections.abc was deprecated starting in Python 3.3+, and in 3.8 it will stop working. The "Astract Base Classes for Collections" module (collections.abc) was introduced in Python 3.3. In previous Python versions, the ABC's were part of the collections module.

Note:
The deprecation warning was not displayed when running tests with tox because nosetests was swallowing the message.

[codecov-io]

Codecov Report

Merging #436 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #436   +/-   ##
=======================================
  Coverage   80.81%   80.81%           
=======================================
  Files          23       23           
  Lines        3941     3941           
  Branches      458      458           
=======================================
  Hits         3185     3185           
  Misses        680      680           
  Partials       76       76

Impacted Files	Coverage Δ
pywt/_utils.py	76.92% <100%> (ø)	⬆️

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[rgommers]

Merged, thanks @cgoldberg