unused variable removed, inheritance fixed

Signed-off-by: adityashrivastav1 <aditya.shri9981@gmail.com>

Author: aditya-shrivastavv

tuf/ngclient/fetcher.py

@@ -17,7 +17,7 @@
 
 
 # Classes
-class FetcherInterface(abc.ABC):
+class FetcherInterface(metaclass=abc.ABCMeta):
     """Defines an interface for abstract network download.
 
     By providing a concrete implementation of the abstract interface,
@@ -28,8 +28,6 @@ class FetcherInterface(abc.ABC):
     The public API of the class is already implemented.
     """
 
-    metaclass = abc.ABCMeta
-
     @abc.abstractmethod
     def _fetch(self, url: str) -> Iterator[bytes]:
         """Fetches the contents of HTTP/HTTPS ``url`` from a remote server.