Bump cryptography from 2.6.1 to 3.2 in /core/admin

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.6.1 to 3.2.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pyca/cryptography/compare/2.6.1...3.2)

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

core/admin/requirements-prod.txt

@@ -5,7 +5,7 @@ bcrypt==3.1.6
 blinker==1.4
 cffi==1.12.3
 Click==7.0
-cryptography==2.6.1
+cryptography==3.2
 decorator==4.4.0
 dnspython==1.16.0
 dominate==2.3.5