python.mk: remove setuptools from RUN_DEPENDS

Historically this was needed as a RUN_DEPENDS due to consumers using
the included pkg_resources, which has been deprecated for some time
now in favour of the importlib suite. (pkg_resources consumers are
strongly advised to migrate to importlib et al.)

PR: 270510
Reviewed by: crees, rm
Tested by: jhale, wen
exp-run by: antoine
Differential Revision: https://reviews.freebsd.org/D39303