Change type of exception raised by values.Formatter

`NotImplementedError` isn't really applicable when the condition is more permanent as it hinges on the specification. A `TypeError` is more correct here, since it's the _type_ of the token that is the issue in these cases.

Author: amn

src/csspring/values.py

@@ -176,7 +176,7 @@ def _(self, production: RepetitionProduction) -> Iterable[str]:
 	def _(self, production: TokenProduction) -> Iterable[str]:
 		if production.attributes:
 			if 'value' not in production.attributes or len(production.attributes) > 1:
-				raise NotImplementedError # the "Values and Units" specification doesn't feature token productions with matching of attributes other than `value`
+				raise TypeError # the "Values and Units" specification doesn't feature token productions with matching of attributes other than `value`
 			yield repr(production.attributes['value'])
 		else:
 			if hasattr(production.type, 'value'):