and do not require sudo.

As in chutney and stem:
- Test all supported python versions
- Test all supported tor versions

Differences between chutney, stem and sbws:
- in sbws we run directly, not an script that calls tox
- we're not using chutney for integration tests (yet) and therefore we're not testing it with different networks
- we don't have shellcheck tests
- we don't support osx nor windows

Closes: #30067

since Travis is failing to check github.com with forbidden error,
but it doesn't fail locally
(https://travis-ci.org/torproject/sbws/jobs/511862070#L1938).

since deb.torproject.org does not support anymore Trusty.

so that CI doesn't fail because of timeout trying to retrieve the
key.

Closes #28922. Bugfix v1.0.3-dev0

Add a tox environment to check the links and call it from travis.
It's not run by tox in order to do not need Internet running tox.
The environment clean is also call from travis, in order to have
sphinx installed when checking the links.

Closes #28670.

converting LaTeX to images.

sbws is getting a significant number of build failures due to keyserver failures.

Using the high-availability keyserver pool reduces the probability of failure.

* Use keyserver pool to avoid single server timeout

travis test failed because keys.gnupg.net timeout.
It should not be the case using the pool.