build(deps): bump securesystemslib[crypto,pynacl] from 0.20.1 to 0.21.0

Bumps [securesystemslib[crypto,pynacl]](https://github.com/secure-systems-lab/securesystemslib) from 0.20.1 to 0.21.0.
- [Release notes](https://github.com/secure-systems-lab/securesystemslib/releases)
- [Changelog](https://github.com/secure-systems-lab/securesystemslib/blob/master/CHANGELOG.md)
- [Commits](secure-systems-lab/securesystemslib@v0.20.1...v0.21.0)

---
updated-dependencies:
- dependency-name: securesystemslib[crypto,pynacl]
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

requirements-pinned.txt

@@ -6,6 +6,6 @@ idna==3.2                 # via requests
 pycparser==2.20           # via cffi
 pynacl==1.4.0             # via securesystemslib
 requests==2.26.0
-securesystemslib[crypto,pynacl]==0.20.1
+securesystemslib[crypto,pynacl]==0.21.0
 six==1.16.0               # via pynacl, securesystemslib
 urllib3==1.26.6           # via requests