Fix requesting negative amounts

Author: nielstron

pycardano/txbuilder.py

@@ -952,6 +952,11 @@ def build(
         requested_amount = Value()
         for o in self.outputs:
             requested_amount += o.amount
+        
+        for pid, m in self.mint.items():
+            for tkn, am in m.items():
+                if am < 0:
+                    requested_amount += Value(multi_asset=MultiAsset({pid: Asset({tkn: -am})}))
 
         # Include min fees associated as part of requested amount
         requested_amount += self._estimate_fee()