support Python 3.9

[davemfish]

This is a requirement for a related invest issue: natcap/invest#1662

We've been using a feature of std library dataclasses that was released in Python 3.10: the kw_only parameter. This parameter avoids the problem, TypeError: non-default argument 'foo' follows default argument, which arises when a dataclass with non-default attributes inherits from one with default attributes. For example,

@dataclass(kw_only=True)
class ArchiveResource(Resource):

    compression: str  # no default value assigned here

Our Requirement:
We like to use default arguments -- attributes with default values assigned on init -- because we like our metadata documents to include all attributes, even ones that haven't yet been assigned a value. That way users editing the yaml by hand can add values without having to know all the attribute names.

Making these non-default arguments "keyword-only" means their positional order no longer matters.

But, in order to make geometamaker compatible with Python 3.9, we need a solution other than using kw_only, which satisfies our requirement. It's possible there might be a solution in Pydantic, which we're already planning to adopt for #29 .