Apply suggestions from code review

Co-authored-by: Nils Homer <[email protected]>

prymer/api/oligo.py

@@ -141,7 +141,8 @@ def __post_init__(self) -> None:
     @property
     def id(self) -> str:
         """Returns the name if there is one, otherwise the span."""
-        return self.name if self.name is not None else str(self.span)
+        return self.name if self.name is not None else f"{self.span}"
+
 
     @property
     def length(self) -> int: