Fix MergeConfigTests.test_collections_merged on Python 3

[rodrigc]

Contributor Checklist:

• [x] There is an associated issue. Create a new one if necessary. Link to it below.
• [x] I have created a newsfragment in src/txkube/newsfragments/
• [x] I have updated the automated tests.

Fixes #183

[codecov-io]

Codecov Report

Merging #184 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #184   +/-   ##
=======================================
  Coverage   97.29%   97.29%           
=======================================
  Files          28       28           
  Lines        2814     2814           
  Branches      274      274           
=======================================
  Hits         2738     2738           
  Misses         48       48           
  Partials       28       28

Impacted Files	Coverage Δ
src/txkube/test/test_network.py	98.32% <100%> (ø)	:arrow_up:

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[rodrigc]

Woops. I got read of 185.misc on this branch