Fix variable annotation for uri in URIMixin.copy_with.

Co-authored-by:  sigmavirus24 <240830+sigmavirus24@users.noreply.github.com>

Author: Sachaa-Thanasius

src/rfc3986/_mixin.py

@@ -404,6 +404,6 @@ def copy_with(
         for key, value in list(attributes.items()):
             if value is misc.UseExisting:
                 del attributes[key]
-        uri: "uri.URIReference" = self._replace(**attributes)
+        uri: _Self = self._replace(**attributes)
         uri.encoding = self.encoding
         return uri