Stream IDs with underscores

[wlerin]

What does this PR do? Why was this PR needed?

Live Stream IDs can contain underscores. This library was rejecting such IDs and as a result failing to finalise any such streams. This PR fixes that.

What are the relevant issue numbers?

This is an extremely minor (single character) change, so I have not created an issue for it. If that is a problem, I will do so.

Does this PR meet the acceptance criteria?

• [ ] Passes flake8 (refer to .travis.yml) (complains about existing linebreaks before "or")
• [ ] Docs are buildable (didn't check)
• [x] Branch has no merge conflicts with master
• [ ] Is covered by a test (would require changing the test media, which I don't have access to)

[coveralls]

Coverage remained the same at 90.988% when pulling 8a78ae4e4b5c526c25bffbb64325d045a7325e0d on wlerin:stream_id into 118fc6c004942fe4d270a2097cce065bd29540c7 on ping:master.