[Security] Bump cryptography from 2.8 to 3.2

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.8 to 3.2. **This update includes a security fix.**
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@2.8...3.2)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

requirements_dev.txt

@@ -6,6 +6,6 @@ flake8==3.7.9
 tox==3.14.3
 coverage==5.0.3
 Sphinx==2.3.1
-cryptography==2.8
+cryptography==3.2
 PyYAML==5.3