Use the correct version of sphinx

doc/build.py

@@ -102,7 +102,9 @@ def build_docs(version='dev', **kwargs):
     raise CalledProcessError(p.returncode, cmd)
   html_dir = os.path.join(work_dir, 'html')
   main_versions = reversed(versions[-3:])
-  check_call(['sphinx-build',
+  # Always use sphinx-build from virtualenv because the system one may be
+  # incompatible with the required version of breathe.
+  check_call([os.path.join('virtualenv', 'bin', 'sphinx-build'),
               '-Dbreathe_projects.format=' + os.path.abspath(doxyxml_dir),
               '-Dversion=' + version, '-Drelease=' + version,
               '-Aversion=' + version, '-Aversions=' + ','.join(main_versions),