onbsru: indicate regex, simplify import

sru/onbsru.py

@@ -2,11 +2,11 @@
 
 import re
 
-from catalogue.sru import almasru as almasru
+from catalogue.sru import almasru
 
 
 class OnbRecordRetriever(almasru.RecordRetriever):
-    pid_re = re.compile('\d{7}')
+    pid_re = re.compile(r'\d{7}')
 
     def __init__(self,
                  subdomain: str = 'obv-at-oenb',