Bump securesystemslib from 0.12.0 to 0.12.1

Bumps [securesystemslib](https://github.com/secure-systems-lab/securesystemslib) from 0.12.0 to 0.12.1.
- [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@v0.12.0...v0.12.1)

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

requirements-pinned.txt

@@ -36,6 +36,6 @@ pathspec==0.6.0
 pycparser==2.19           # via cffi
 pynacl==1.3.0             # via securesystemslib
 python-dateutil==2.8.0
-securesystemslib[crypto,pynacl]==0.12.0
+securesystemslib[crypto,pynacl]==0.12.1
 six==1.12.0
 subprocess32==3.5.4 ; python_version < "3"