Avoid instantiating deserialize_any for KeyClassifier

Deserialize_str involves compiling less code.
diff --git a/src/value/de.rs b/src/value/de.rs
index f8cc2b8..1749f26 100644
--- a/src/value/de.rs
+++ b/src/value/de.rs
@@ -1301,7 +1301,7 @@
     where
         D: serde::Deserializer<'de>,
     {
-        deserializer.deserialize_any(self)
+        deserializer.deserialize_str(self)
     }
 }