Fix flake8 F632 error

[at-wat]

Master branch and PR builds are failing due to the following flake8 error.

https://travis-ci.org/ros-infrastructure/rosdep/jobs/490897551#L546

/home/travis/build/ros-infrastructure/rosdep/src/rosdep2/main.py:350:16: F632 use ==/!= to compare str, bytes, and int literals
            if key is 'source':
               ^
flake8 reported 1 errors

[codecov-io]

Codecov Report

Merging #659 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master     #659   +/-   ##
=======================================
  Coverage   75.19%   75.19%           
=======================================
  Files          30       30           
  Lines        2890     2890           
=======================================
  Hits         2173     2173           
  Misses        717      717

Impacted Files	Coverage Δ
src/rosdep2/main.py	48.49% <0%> (ø)	⬆️

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[wjwwood]

lgtm, thanks! Merging despite the codecov failure.