Bump cryptography from 2.7 to 3.2

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.7 to 3.2.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@2.7...3.2)

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

requirements.txt

@@ -22,7 +22,7 @@ coreapi==2.3.3
 coreapi-cli==1.0.9
 coreschema==0.0.4
 coverage==4.5.3
-cryptography==2.7
+cryptography==3.2
 daphne==2.3.0
 defusedxml==0.6.0
 dj-database-url==0.5.0