test/contrib: Fix invalid escapes in regex strings

Flagged by flake8 v3.6.0, as W605, plus a few others identified
incidentally, e.g. 59ffecf66cf4d08c4b431e457b083878d66a3fd6.

Note that r"\n" matches to "\n" under re.match/search.

contrib/seeds/generate-seeds.py

@@ -74,7 +74,7 @@ def name_to_ipv6(addr):
         raise ValueError('Could not parse address %s' % addr)
 
 def parse_spec(s, defaultport):
-    match = re.match('\[([0-9a-fA-F:]+)\](?::([0-9]+))?$', s)
+    match = re.match(r'\[([0-9a-fA-F:]+)\](?::([0-9]+))?$', s)
     if match: # ipv6
         host = match.group(1)
         port = match.group(2)
@@ -136,4 +136,3 @@ def main():
 
 if __name__ == '__main__':
     main()
-