Bump securesystemslib[crypto,pynacl] from 0.14.2 to 0.15.0

Bumps [securesystemslib[crypto,pynacl]](https://github.com/secure-systems-lab/securesystemslib) from 0.14.2 to 0.15.0.
- [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@v0.14.2...v0.15.0)

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

requirements-pinned.txt

@@ -35,6 +35,6 @@ pathspec==0.8.0
 pycparser==2.20           # via cffi
 pynacl==1.3.0             # via securesystemslib
 python-dateutil==2.8.1
-securesystemslib[crypto,pynacl]==0.14.2
+securesystemslib[crypto,pynacl]==0.15.0
 six==1.15.0
 subprocess32==3.5.4       # via securesystemslib