Fix flake8-bugbear warning

B027 `BaseSpecifier.prereleases` is an empty method in an
     abstract base class, but has no abstract decorator

src/packaging/specifiers.py

@@ -82,6 +82,7 @@ def prereleases(self) -> Optional[bool]:
         """
 
     @prereleases.setter
+    @abc.abstractmethod
     def prereleases(self, value: bool) -> None:
         """Setter for :attr:`prereleases`.