use string for seed if none provided

When calling randomtable without providing a seed, the warning `DeprecationWarning: Seeding based on hashing is deprecated since Python 3.9 and will be removed in a subsequent version. The only supported seed types are: None, int, float, str, bytes, and bytearray.` is emitted.

petl/util/random.py

@@ -154,7 +154,7 @@ def __init__(self, numrows=100, fields=None, wait=0, seed=None):
         else:
             self.fields = OrderedDict(fields)
         if seed is None:
-            self.seed = datetime.datetime.now()
+            self.seed = datetime.datetime.now().isoformat()
         else:
             self.seed = seed