Mostly SCC tests

- Add tests for SCC reading and writing.
- Add test mixin for comparing parsed CaptionSets with some fuzzy matching.
- Fix a bug in SCC writing where spaces did not convert correctly.
- Fix a bug in SCC writing where unicode text was not properly handled.
- Caption: add a utility function to get caption text, useful for testing.
- Setup: change required versions to be a minimum check rather than equality check.
- Change the default language for SCC reading to be en-US, to match other formats.