Pycaption

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- Implemented time shift for WebVTTReader
- Removed WebVTTWriter 'start' position alignment
- Updated the SCC Pop-On caption timing logic
- Fixed the correction of end times for multiple last captions
- Fixed bug when flushing implicit buffers and old key was None

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- Implemented Tab Offset commands for SCCReader
- Implemented caption safe area limits (80% horizontally and 90% vertically)
- Implemented SCC translator

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- Added newline between merged SRT captions with overlapping timestamps
- Updated tests for SAMI format
- Updated tests for SRT format
- Added zero padding to 1-digit hours outputted by WebVTTWriter

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- Dropped support for Python 3.5
- Updated tests to run using pytest
- Added pre-commit config

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- Fixed issue with SCC paint-on buffer not being cleared after storing
- Removed null DFXPReader captions from the resulting caption list
- Updated SCCReader double command handling to include the positioning and tab offset case

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- Added MicroDVD format
- Fix for missing end times when reading multiple SAMI paragraphs inside a SYNC
- Fix for wrong order when multiple SRT captions have the same timestamp
- Fix for DFXP timestamps adding leading zeros to 2-digit hours
- Added support for BeautifulSoup 4.9
- Added tests for SCC to DFXP conversion when the source contains ampersands
- Added support for Python 3.9