Bump securesystemslib from 0.16.0 to 0.17.0

We just released securesystemslib 0.17.0 and it would be better if tuf did
not pin the prior version.

https://pypi.org/project/securesystemslib/0.17.0/
https://github.com/secure-systems-lab/securesystemslib/releases/tag/v0.17.0

Signed-off-by: Joshua Lock <jlock@vmware.com>

Author: joshuagl

docs/CHANGELOG.md

@@ -14,7 +14,8 @@
 * Print a warning in `tuf.repository_tool` when metadata is written without a
   signature (#1100)
 * Remove iso8661 dependency (#1176)
-* Bump dependencies: cffi (#1146), cryptography (#1149), urllib (#1179)
+* Bump dependencies: cffi (#1146), cryptography (#1149), urllib (#1179),
+  securesystemslib (#1183)
 * Overhauled logging to be less verbose and less alarming, by removing logging
   in the library when an exception is raised (including the same information
   that was logged) and using more appropriate log levels (#1145)

requirements-pinned.txt

@@ -7,9 +7,8 @@ idna==2.10                 # via requests
 ipaddress==1.0.23         ; python_version < '3' # via cryptography
 pycparser==2.20           # via cffi
 pynacl==1.4.0             # via securesystemslib
-python-dateutil==2.8.1    # via securesystemslib
 requests==2.24.0
-securesystemslib[crypto,pynacl]==0.16.0
+securesystemslib[crypto,pynacl]==0.17.0
 six==1.15.0
 subprocess32==3.5.4       ; python_version < '3' # via securesystemslib
 urllib3==1.25.11           # via requests