Fix import for Python 3.9+

Author: shadchin

Diff for: pydub/pyaudioop.py

@@ -6,7 +6,10 @@
     from builtins import min as builtin_min
 import math
 import struct
-from fractions import gcd
+try:
+    from fractions import gcd
+except ImportError:  # Python 3.9+
+    from math import gcd
 from ctypes import create_string_buffer