python/tests/test_source.py: fix format for python3

diff --git a/python/tests/test_source.py b/python/tests/test_source.py
index 208897b..3358d34 100755 (executable)
--- a/python/tests/test_source.py
+++ b/python/tests/test_source.py
@@ -59,7 +59,7 @@ class aubio_source_read_test_case(aubio_source_test_case_base):
         try:
             f = source(soundfile, samplerate, hop_size)
         except RuntimeError as e:
-            self.skipTest('failed opening with hop_s = {:d}, samplerate = {:d} ({:s})'.format(hop_size, samplerate, e))
+            self.skipTest('failed opening with hop_s = {:d}, samplerate = {:d} ({:s})'.format(hop_size, samplerate, str(e)))
         assert f.samplerate != 0
         self.read_from_source(f)