sysutils/py-supervisor: Update to 4.2.1

Changelog for 4.2.1:

• Fixed a bug on Python 3 where a network error could cause supervisord to crash with the error <class 'TypeError'>:can't concat str to bytes.
• Fixed a bug where a test would fail on systems with glibc 2.3.1 because the default value of SOMAXCONN changed.

PR: 251728
Reviewed by: osa (mentor)
Approved by: osa (mentor), thomas@gibfest.dk (maintainer)
Differential Revision: https://reviews.freebsd.org/D27686