devel/ghpr: add missing BUILD_DEPENDS

This change adds additional build dependencies accidentally missed in
the previous commit.

Although the devel/py-setuptools dependency is technically redundant in
light of devel/py-setuptools-scm, it's best to express all of the needed
dependencies in BUILD_DEPENDS.

Reported by: poudriere
Approved by: imp (maintainer)
Pointyhat to: ngie
Fixes: 59337d86174 ("devel/ghpr: update to v3")
Differential Revision: https://reviews.freebsd.org/D55962