Avoid TypeError in Transaction.__repr__ (#703)

Only add isolation to repr if it's not None

Author: BeatButton

asyncpg/transaction.py

@@ -234,7 +234,8 @@ def __repr__(self):
         attrs = []
         attrs.append('state:{}'.format(self._state.name.lower()))
 
-        attrs.append(self._isolation)
+        if self._isolation is not None:
+            attrs.append(self._isolation)
         if self._readonly:
             attrs.append('readonly')
         if self._deferrable: