Allow reproducible build

With this patch, building the Debian package is now predictible
and always the same. See Debian bug report here:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=799206

Author: thomasgoirand

setup.py

@@ -57,7 +57,7 @@ def get_version():
 
 # Add all optional dependencies to testing requirements.
 test_all = []
-for name, requirements in extras_require.items():
+for name, requirements in sorted(extras_require.items()):
     test_all += requirements
 extras_require['test_all'] = test_all