devel/py-sysctl: Update to 0.3.0

Bring py-sysctl to 0.3.0, which features the following changes:

• .value now lazy-loaded on first access to avoid the overhead of getting unused values

• .description field added, also lazy-loaded on first access to minimize overhead

• Support for CTL_SYSCTL_NEXTNOSKIP to include sysctls marked with CTLFLAG_SKIP once D26560 lands

Reviewed by: wg
Approved by: wg (ports, maintainer)
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D26684