chore(deps): bump securesystemslib from 0.30.0 to 0.31.0 (#570)

Bumps [securesystemslib](https://github.com/secure-systems-lab/securesystemslib) from 0.30.0 to 0.31.0.
- [Release notes](https://github.com/secure-systems-lab/securesystemslib/releases)
- [Changelog](https://github.com/secure-systems-lab/securesystemslib/blob/main/CHANGELOG.md)
- [Commits](secure-systems-lab/securesystemslib@v0.30.0...v0.31.0)

---
updated-dependencies:
- dependency-name: securesystemslib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-test.txt

@@ -1,5 +1,5 @@
 iso8601==2.1.0
 requests==2.31.0
-securesystemslib==0.30.0
+securesystemslib==0.31.0
 six==1.16.0
 tuf==3.1.0