REF: make Defaults a singleton (#218) (#1136)

Co-authored-by: JHM Darbyshire (M1) <[email protected]>

Author: attack68

python/rateslib/_spec_loader.py

@@ -90,7 +90,7 @@ def _get_kwargs(spec: str) -> dict[str, Any]:
             d["roll"] = _map_str_int(d["roll"])
         if "leg2_roll" in d:
             d["leg2_roll"] = _map_str_int(d["leg2_roll"])
-        return d
+        return d  # type: ignore[return-value]
 
     INSTRUMENT_SPECS = {k: _get_kwargs(k) for k in df.columns[4:]}