Avoid deadlock with subprocess

[coreywright]

Heed Python's subprocess module documentation warnings about deadlock
when using PIPE for a Popen object's stderr with Popen.wait() and
Popen.stderr.read() and use Popen.communicate() instead.

Fixes #126.

[coreywright]

the pep8 violation ./tests/test_imports.py:13:1: E305 expected 2 blank lines after class or function definition, found 1 is a "pre-existing condition" (disclaimer: not covered under the terms of this PR's insurance policy), but resolved by #127.

[codecov-io]

Codecov Report

Merging #128 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #128   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         392    391    -1     
  Branches       82     82           
=====================================
- Hits          392    391    -1

Impacted Files	Coverage Δ
supernova/supernova.py	100% <100%> (ø)	⬆️

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[coreywright]

force pushed up a new commit to clarify where these warnings, as referenced in the commit message, exist (ie Python's subprocess module documentation).