Python 2 -> 3
M3U8: general Python improvements
M3U8: follow PEP-8 (long lines)
M3U8: make a few magic strings constant
M3U8: `Parser` class is not needed

Since `Parser` is stateless and the `m3u8` package defines its own
namespace already, this class is not needed.
M3U8: `_parse_line` -> `_get_line_tagname_attributes`
M3U8: minor improvements
M3U8: add basic docstrings