builtin: remove spack.variant imports (#50576)

Signed-off-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>

lib/spack/spack/variant.py

@@ -208,7 +208,7 @@ def variant_type(self) -> VariantType:
         else:
             return VariantType.SINGLE
 
-    def __str__(self):
+    def __str__(self) -> str:
         return (
             f"Variant('{self.name}', "
             f"default='{self.default}', "
@@ -491,14 +491,14 @@ class DisjointSetsOfValues(collections.abc.Sequence):
         *sets (list): mutually exclusive sets of values
     """
 
-    _empty_set = set(("none",))
+    _empty_set = {"none"}
 
-    def __init__(self, *sets):
+    def __init__(self, *sets: Tuple[str, ...]) -> None:
         self.sets = [set(_flatten(x)) for x in sets]
 
         # 'none' is a special value and can appear only in a set of
         # a single element
-        if any("none" in s and s != set(("none",)) for s in self.sets):
+        if any("none" in s and s != {"none"} for s in self.sets):
             raise spack.error.SpecError(
                 "The value 'none' represents the empty set,"
                 " and must appear alone in a set. Use the "