better documentation

[rogpeppe]

Very few of the types and methods in pymacaroons have documentation. This is problematic, because it's not always clear what types the arguments are expected to be or what the methods do. For example, the caveat_id member of the public Caveat type is expected to be an instance of bytes (I think), but it's not clear that is the case.

[BigSamu]

Support this query. Would be great to have a more detailed documentation if possible

[miketheman]

Thinking about type hints, the currently advertised support for Python versions would have to be narrowed to support the right kinds of type hints.

Specifically, Python 2.7-3.7 are all unsupported versions of Python - should those version support be removed so newer syntax and type hints can be added?