Update dependency cachetools to v3 (#357)

diff --git a/setup.py b/setup.py
index 7149981..60c7df4 100644
--- a/setup.py
+++ b/setup.py
@@ -19,7 +19,7 @@
 
 
 DEPENDENCIES = (
-    'cachetools>=2.0.0,<3.0',
+    'cachetools>=2.0.0,<3.2',
     'pyasn1-modules>=0.2.1',
     'rsa>=3.1.4,<4.1',
     'setuptools>=40.3.0',