Bug 1252294 - Add python/futures to sys.path when building Sphinx docs; r=chmanchester

RTD is complaining that it can't find concurrent.futures. We solve this
problem on RTD by adjusting sys.path in the Sphinx config file to add
paths to the vendored packages.

Sadly, there isn't really an easy way to test this before landing. But
it should work.

MozReview-Commit-ID: FrsLrbbOQWt

--HG--
extra : rebase_source : 4debf9b1902e26babe7e9ddaee8142a14674d393

tools/docs/conf.py

@@ -15,6 +15,7 @@ OUR_DIR = os.path.dirname(__file__)
 topsrcdir = os.path.normpath(os.path.join(OUR_DIR, '..', '..'))
 
 EXTRA_PATHS = (
+    'python/futures',
     'python/jsmin',
     'python/mach',
     'python/mozbuild',