comms/py-pyserial: Add USE_PYTHON=concurrent

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Summary

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: D30920
MFH:		2021Q2 (ports compliance)

Test Plan

• portlint: OK (looks fine.)
• testport: OK (poudriere: 140017, [armv7,aarch64,i386,amd64], tested)
• unittest: OK (

tested on aarch64:
Python3.6: Ran 15 tests in 17.432s OK
Python3.7: Ran 15 tests in 17.303s OK
Python3.8: Ran 15 tests in 17.405s OK
Python3.9: Ran 15 tests in 17.456s OK
)

[koobs]

Technical Approved by: portmgr (blanket: ports / python compliance) but if you'd like to wait for @sbz that's fine :)

[loader]

Thanks @koobs, @sbz.