devel/py-setproctitle: Fix conflict with concurrent Python flavors

Since devel/py-setproctitle installs HISTORY.rst and README.rst in a
fixed location, pkg won't allow multiple flavors installed at the same time
and concurrent ports conflict on install with each other.

This change adds USE_PYTHON=concurrent which compensates for files in
DOCSDIR automatically, by using a different directory for each Python
version.

PR: 238601
Submitted by: Ralf van der Enden <tremere cainites net>
Approved by: portmgr (blanket(s): just fix it, ports (python) compliance)
MFH: 2019Q2 (blanket: bugfix)