Remove unnecessary coerce to list

[jdufresne]

sorted() takes any iterable and always returns a new list. No need to
eagerly coerce to a list.

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Does travis tests pass with this change (enable it first in your forked repo and wait for the travis build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?

[codecov-io]

Codecov Report

Merging #1321 into master will not change coverage by %.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1321   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files          19       19           
  Lines        6533     6533           
=======================================
  Hits         6064     6064           
  Misses        469      469           

Impacted Files	Coverage Δ
tests/test_commands.py	99.94% <100.00%> (ø)

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[andymccurdy]

looks good to me, thanks!