Use datetime.__str__(), not isoformat()

Use datetime.__str__(), not isoformat() to conform to the native
timestamp format of SQLite3

lib/nexrad/storm.py

@@ -199,14 +199,12 @@ class StormReport(DatabaseTable):
             })
 
         if timestamp is not None:
-            clauses.append("timestamp_start >= :timestamp and timestamp_end <= :timestamp")
+            clauses.append(":timestamp between timestamp_start and timestamp_end")
 
             values.update({
-                'timestamp': timestamp.isoformat()
+                'timestamp': str(timestamp)
             })
 
-            values.append(timestamp.isoformat())
-
         sql = "select " + ", ".join(columns) + " from nexrad_storm_report"
 
         if len(clauses) > 0: