comms/py-pyserial: Add USE_PYTHON=concurrent

• Allow concurrent installation for bin/pyserial-{miniterm,ports}

Reviewed by: koobs, sbz
Approved by: koobs (ports, mentor), sbz (maintainer)
Differential Revision: https://reviews.freebsd.org/D30920
MFH: 2021Q2 (ports compliance)

(cherry picked from commit 6edd4051890c2ea511edfc8da19dfc07d5933a3f)