redis import failure on python2 causes crash

Don't use ModuleNotFoundError, it's python 3.6+. Use plain ImportError
instead.

Author: rouilj

roundup/backends/back_anydbm.py

@@ -37,7 +37,7 @@
 
 try:
     from roundup.backends import sessions_redis
-except ModuleNotFoundError:
+except ImportError:
     sessions_redis = None
 
 from roundup.backends.indexer_common import get_indexer

roundup/backends/back_sqlite.py

@@ -17,7 +17,7 @@
 
 try:
     from roundup.backends import sessions_redis
-except ModuleNotFoundError:
+except ImportError:
     sessions_redis = None
 
 from roundup.anypy.strings import uany2s