Add type hints

[Conengmo]

• [x] Add type hints to all modules in Branca.
• [x] Make sure Mypy passes.

Contains some functional changes as well:

• Alter _parse_hex to output RGBA floats directly. We only use that function inside of _parse_color, which needs RGBA floats as output.
• No longer have bytes as accepted type for color strings. That's a remnant from the Python 2 times.
• Merge duplicate code in Link, JavascriptLink and CssLink.

Do separately later:

• make subclasses of Figure use the same types for width and height. Also a separate PR I think. Would be easiest to have _parse_size just return a single string.
• simplify the _camelify function. Is it even needed?
• remove get_templates