Bump cryptography from 2.1.4 to 2.3 in /setup

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.1.4 to 2.3.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@2.1.4...2.3)

Signed-off-by: dependabot[bot] <support@github.com>

setup/requirements.txt

@@ -8,7 +8,7 @@ cffi==1.11.2
 chardet==3.0.4
 cheroot==5.10.0
 CherryPy==12.0.1
-cryptography==2.1.4
+cryptography==2.3
 cycler==0.10.0
 enum34==1.1.6
 functools32==3.2.3.post2