Add a tox environment for downstream testing

[exarkun]

https://twistedmatrix.com/trac/ticket/9042

[codecov-io]

Codecov Report

Merging #710 into trunk will increase coverage by 1.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            trunk     #710      +/-   ##
==========================================
+ Coverage   89.99%   91.26%   +1.27%     
==========================================
  Files         841      844       +3     
  Lines      146544   147605    +1061     
  Branches    12848    13061     +213     
==========================================
+ Hits       131884   134718    +2834     
+ Misses      12291    10643    -1648     
+ Partials     2369     2244     -125

[rodrigc]

I would say instead of calling it downstream, just call it treq. Each time you add a new downstream build, just call it what you are testing.

Be warned that Travis can get overloaded, so adding more builds there should be done with
caution.
Adding builds like this to the buildbot cluster and making them "required" for the overall build to succeed may be more manageable in terms of overall resources.

[hawkowl]

I think this shouldn't gate the build right now, as it does fail. The external state concern of mine is secondary. I think, make it non-dependent on passing for now (with the regression fix moving it out of allowed-failures), and merge.

[hawkowl]

I think we should not make Treq able to fail the build; it relies on external websites (httpbin?) for its integration testing. Of course, that hasn't been an issue on treq development, so... use your discretion.

[glyph]

Maybe we should get rid of that particular misfeature of treq's CI?

[hawkowl]

@glyph Maybe. I don't know how we'll do that in the short term, though :)

[glyph]

Closing PR due to staleness and merge conflicts.