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>

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: