Use tool-specific user-agent to retrieve custom rules from websites

[markpaters]

Fixes: #774
gitlab.com (and possibly other sites) reject urlopen() requests from the default user-agent used by urllib2. This PR adds a tool-specific user-agent in the request header for http and https when retrieving custom rules files.
With thanks to Christophe Bedard, who correctly identified the problem.

[codecov-commenter]

Codecov Report

Merging #775 into master will increase coverage by 0.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
+ Coverage   74.84%   74.86%   +0.01%     
==========================================
  Files          41       41              
  Lines        3220     3226       +6     
==========================================
+ Hits         2410     2415       +5     
- Misses        810      811       +1     

Impacted Files	Coverage Δ
src/rosdep2/sources_list.py	86.58% <85.71%> (-0.07%)	⬇️

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[markpaters]

This is a very small change. I'd much appreciate it if someone were willing to review it.