fix: support dumbdbm on windows.

Make db_exists in back_anydbm.py can detect a dumbdbm by looking
for the .dat file.

Author: rouilj

roundup/backends/back_anydbm.py

@@ -53,7 +53,7 @@
 
 def db_exists(config):
     # check for the user db
-    for db in 'nodes.user nodes.user.db'.split():
+    for db in 'nodes.user nodes.user.db nodes.user.dat'.split():
         if os.path.exists(os.path.join(config.DATABASE, db)):
             return 1
     return 0