Riase required `attrs` version to version

[BPHarris]

The minimum version is set to 21 this is incorrect.

• we require the alias parameter for attrs.field which was added in version 22 per the docs
• we require that the frozen symbol models be hashable, this is the default case in version 23, so either we must bump to this version or explicitly set the symbols to be hashable (the latter is untested but presumably works)