Rename the reward redeemer tag to withdrawal (Python-Cardano#312)

Ogmios 5 and blockfrost both refer to script redeemers that use the
withdrawal redeemer as "withdrawal:{index}". The only place where the
name of the redeemer tag is used is for evaluating the execution steps
in the transaction builder. Therefore I recommend renaming the redeemer
tag to the name used by other cardano tooling.

Note that in Ogmios 6, this name changes to "withdraw"

pycardano/plutus.py

@@ -932,7 +932,7 @@ class RedeemerTag(CBORSerializable, Enum):
     SPEND = 0
     MINT = 1
     CERT = 2
-    REWARD = 3
+    WITHDRAWAL = 3
 
     def to_primitive(self) -> int:
         return self.value