FIX: Fix ImportError for gcd (#573)

* FIX: Fix ImportError for `gcd`

* add testing agains python=3.9

* Revert "add testing agains python=3.9"

This reverts commit 46e6f3c.

Co-authored-by: mmcky <[email protected]>

Author: oyamad

quantecon/graph_tools.py

@@ -5,7 +5,7 @@
 import numpy as np
 from scipy import sparse
 from scipy.sparse import csgraph
-from fractions import gcd
+from math import gcd
 from numba import jit
 
 from .util import check_random_state

quantecon/markov/core.py

@@ -80,7 +80,7 @@
 
 """
 import numbers
-from fractions import gcd
+from math import gcd
 import numpy as np
 from scipy import sparse
 from numba import jit