Update securesystemslib dependency to 0.18.0

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

requirements-pinned.txt

@@ -8,7 +8,7 @@ ipaddress==1.0.23         ; python_version < '3' # via cryptography
 pycparser==2.20           # via cffi
 pynacl==1.4.0             # via securesystemslib
 requests==2.24.0
-securesystemslib[crypto,pynacl]==0.17.0
+securesystemslib[crypto,pynacl]==0.18.0
 six==1.15.0
 subprocess32==3.5.4       ; python_version < '3' # via securesystemslib
 urllib3==1.25.11           # via requests

setup.py

@@ -116,7 +116,7 @@
   install_requires = [
     'requests>=2.19.1',
     'six>=1.11.0',
-    'securesystemslib>=0.16.0'
+    'securesystemslib>=0.18.0'
   ],
   tests_require = [
     'mock; python_version < "3.3"'