Metadata API: Make Metadata constructor Generic

In from_dict() we cannot at static type check time tell what the
specific signed type so have to cast, but the constructor can be
properly Generic so in the following md is automatically annotated
as "Metadata[Root]" and md.signed as "Root":

    root = Root(...)
    md = Metadata(root, OrderedDict())

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tuf/api/metadata.py

@@ -77,11 +77,8 @@ class Metadata(Generic[T]):
             signing the canonical serialized representation of 'signed'.
     """
 
-    def __init__(
-        self, signed: "Signed", signatures: "OrderedDict[str, Signature]"
-    ):
-        # init does not verify type constraint T: only from_bytes() does
-        self.signed: T = cast(T, signed)
+    def __init__(self, signed: T, signatures: "OrderedDict[str, Signature]"):
+        self.signed: T = signed
         self.signatures = signatures
 
     @classmethod
@@ -127,7 +124,8 @@ def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata":
             signatures[sig.keyid] = sig
 
         return cls(
-            signed=inner_cls.from_dict(metadata.pop("signed")),
+            # Specific type T is not known at static type check time: cast
+            signed=cast(T, inner_cls.from_dict(metadata.pop("signed"))),
             signatures=signatures,
         )