setup.py: require cryptography instead of rsa

The rsa library is slower and not as well-maintained as the cryptography
library. Require the cryptography library instead of rsa.

On Python 2.7, Pip users will install cryptography-3.3.2, because that
is the latest cryptography version that advertises support for Python
2.7.

Author: ktdreyer

setup.py

@@ -22,10 +22,8 @@
 DEPENDENCIES = (
     "cachetools>=2.0.0,<5.0",
     "pyasn1-modules>=0.2.1",
-    # rsa==4.5 is the last version to support 2.7
-    # https://github.com/sybrenstuvel/python-rsa/issues/152#issuecomment-643470233
-    'rsa<4.6; python_version < "3.6"',
-    'rsa>=3.1.4,<5; python_version >= "3.6"',
+    # cryptography==3.3.X is the last version to support 2.7
+    'cryptography',
     # install enum34 to support 2.7. enum34 only works up to python version 3.3.
     'enum34>=1.1.10; python_version < "3.4"',
     "six>=1.9.0",