Mark a regex pattern as a raw string

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/policies/autopkgtest.py

@@ -529,7 +529,7 @@ class AutopkgtestPolicy(BasePolicy):
         # serves no purpose. Just check some key packages which actually use
         # gcc during the test, and libreoffice as an example for a libgcc user.
         if src.startswith('gcc-'):
-            if re.match('gcc-\d$', src):
+            if re.match(r'gcc-\d$', src):
                 # add gcc's own tests, if it has any
                 srcinfo = source_suite.sources[src]
                 if 'autopkgtest' in srcinfo.testsuite: