lang/hermes: Make Python an explicit build dependency (D29353)

This unbreaks the build when using devel/samurai.

It previously was available in the configure phase through ninja
which has a run dependency on Python.

<phase: configure >

> Missing "/usr/local/bin/python3.7" to create a binary alias at "/wrkdirs/usr/ports/lang/hermes/work/.bin/python"

http://package18.nyi.freebsd.org/data/122amd64-default-foo/2021-04-04_07h37m38s/logs/errors/hermes-javascript-engine-0.7.2.20210322.log

PR: 254678