Use Generic type T in return types

Modify the return type of the Metadata factory
methods (from_*) to use the Generic type T. This helps IDEs
understand the return type and give proper hints.

The static method from_bytes() has to become a class method
to get the needed Generic info.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/api/metadata.py

@@ -80,7 +80,7 @@ def __init__(self, signed: T, signatures: "OrderedDict[str, Signature]"):
         self.signatures = signatures
 
     @classmethod
-    def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata":
+    def from_dict(cls, metadata: Dict[str, Any]) -> "Metadata[T]":
         """Creates Metadata object from its dict representation.
 
         Arguments:
@@ -133,7 +133,7 @@ def from_file(
         filename: str,
         deserializer: Optional[MetadataDeserializer] = None,
         storage_backend: Optional[StorageBackendInterface] = None,
-    ) -> "Metadata":
+    ) -> "Metadata[T]":
         """Loads TUF metadata from file storage.
 
         Arguments:
@@ -160,11 +160,12 @@ def from_file(
         with storage_backend.get(filename) as file_obj:
             return cls.from_bytes(file_obj.read(), deserializer)
 
-    @staticmethod
+    @classmethod
     def from_bytes(
+        cls,
         data: bytes,
         deserializer: Optional[MetadataDeserializer] = None,
-    ) -> "Metadata":
+    ) -> "Metadata[T]":
         """Loads TUF metadata from raw data.
 
         Arguments: