chore(models/Report): fix reportIncompatibleMethodOverride lints

This check is currently turned off in our `basic` `typeCheckingMode`
profile, but is enabled in the `standard` profile.

Things done here:

- matched the signature of `default` with `JSONEncoder.default`

Error:

    unblob/models.py
      unblob/models.py:258:9 - error: Method "default" overrides class "JSONEncoder" in an incompatible manner
        Parameter 2 name mismatch: base parameter is named "o", override parameter is named "obj" (reportIncompatibleMethodOverride)

Author: vlaci

unblob/models.py

@@ -255,7 +255,8 @@ def get_output_dir(self) -> Optional[Path]:
 
 
 class _JSONEncoder(json.JSONEncoder):
-    def default(self, obj):
+    def default(self, o):
+        obj = o
         if attrs.has(type(obj)):
             extend_attr_output = True
             attr_output = attrs.asdict(obj, recurse=not extend_attr_output)