Require cryptography >= 2.3

This is in response to https://nvd.nist.gov/vuln/detail/CVE-2018-10903

Author: Landon Gilbert-Bland

requirements.txt

@@ -5,7 +5,7 @@ certifi==2018.4.16
 cffi==1.11.5
 chardet==3.0.4
 click==6.7
-cryptography==2.2.2
+cryptography==2.3
 docutils==0.14
 Flask==1.0
 Flask-Sphinx-Themes==1.0.2

setup.py

@@ -34,7 +34,7 @@
           'PyJWT',
       ],
       extras_require={
-        'asymmetric_crypto':  ["cryptography"]
+        'asymmetric_crypto':  ["cryptography >= 2.3"]
       },
       classifiers=[
         'Development Status :: 5 - Production/Stable',