Bump cryptography from 2.4.2 to 3.3.2 in /docs

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.4.2 to 3.3.2.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/main/CHANGELOG.rst)
- [Commits](pyca/cryptography@2.4.2...3.3.2)

---
updated-dependencies:
- dependency-name: cryptography
  dependency-type: direct:production
...

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

docs/requirements.txt

@@ -14,7 +14,7 @@ click==6.7
 colorama==0.3.7
 configparser==3.7.1
 coverage==4.1
-cryptography==2.4.2
+cryptography==3.3.2
 docker==2.5.1
 docker-pycreds==0.4.0
 docopt==0.6.2