build(deps): bump cryptography from 3.0 to 3.1

Bumps [cryptography](https://github.com/pyca/cryptography) from 3.0 to 3.1.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](pyca/cryptography@3.0...3.1)

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

requirements-pinned.txt

@@ -2,7 +2,7 @@ certifi==2020.6.20       # via requests
 cffi==1.14.2              # via cryptography, pynacl
 chardet==3.0.4            # via requests
 colorama==0.4.3           # via securesystemslib
-cryptography==3.0         # via securesystemslib
+cryptography==3.1         # via securesystemslib
 enum34==1.1.6             ; python_version < '3' # via cryptography
 idna==2.10                 # via requests
 ipaddress==1.0.23         ; python_version < '3' # via cryptography