data: Posting.is_payment casts threshold to address typing issue.

conservancy_beancount/data.py

@@ -25,6 +25,7 @@ import decimal
 from beancount.core import account as bc_account
 
 from typing import (
+    cast,
     Iterable,
     Iterator,
     MutableMapping,
@@ -212,13 +213,11 @@ class Posting(BasePosting):
     meta: Metadata  # type:ignore[assignment]
 
     def is_payment(self, threshold: DecimalCompat=0) -> bool:
+        threshold = cast(decimal.Decimal, threshold)
         return (
             self.account.is_real_asset()
             and self.units.number is not None
-            # mypy says abs returns an object and we can't negate that.
-            # Since we know threshold is numeric, it seems safe to assume
-            # the return value of abs is numeric as well.
-            and self.units.number < -abs(threshold)  # type:ignore[operator]
+            and self.units.number < -abs(threshold)
         )