Fix version string for npm by decoding bytes

[christophebedard]

Before:

$ rosdep --all-versions
0.20.1

Versions of installers:
  apt-get 2.0.5
  pip 21.0.1
  setuptools 53.0.0
  gem 3.1.2
  npm b'6.13.4'

After:

$ rosdep --all-versions
0.20.1

Versions of installers:
  apt-get 2.0.5
  pip 21.0.1
  setuptools 53.0.0
  gem 3.1.2
  npm 6.13.4

Signed-off-by: Christophe Bedard [email protected]

[codecov-commenter]

Codecov Report

Merging #814 (60eeebd) into master (30a25a7) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #814   +/-   ##
=======================================
  Coverage   74.96%   74.96%           
=======================================
  Files          42       42           
  Lines        3283     3283           
=======================================
  Hits         2461     2461           
  Misses        822      822           

Impacted Files	Coverage Δ
src/rosdep2/platforms/npm.py	79.54% <0.00%> (ø)

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[nuclearsandwich]

Thanks!