Short-con

- Enhanced `constants()` to support the creation of non-frozen
dataclass instances.

- Added `dc()` to support the creation of quick-and-dirty dataclasses.

- Switched from attrs to dataclass.

- Dropped support for Pythons earlier than 3.7.

- Reworked the API to prioritize convenience: `cons()` and `enumcons()` offer
default behavior as easily as possible; and `constants()` allows the user to
control the name of the underlying dataclass (a required parameter in v1) or
to supply a callable to compute each value from its name (similar behavior
was in v1).

- Changed signature to `cons(_clsname, **kwargs)` to avoid conflict when a
user's kwargs needs `name` as a key.

- README improvements.

- Added support for relevant read-only dict behaviors.

- Added docstrings.

- README improvements.