Remove Python dependency on testtools in Travis

All the Travis builds stopped working yesterday due to a setup step requiring input.
Removing the offending Python dependency fixes the problem and still yields running tests.

Example Travis build log from https://travis-ci.org/web-animations/web-animations-js/builds/18178877
```
+ pip install -r tools/python/requirements.txt --use-mirrors
Downloading/unpacking testtools>=0.9.30 (from -r tools/python/requirements.txt (line 1))
User for c.pypi.python.org: 
No output has been received in the last 10 minutes, this potentially indicates a stalled build or something wrong with the build itself.
The build has been terminated
```