Enable tests for security/py-artifacts with MODPY_PYTEST

While testing the py-yapf update I saw that the py-artifacts tests
don't run. You get the "0 tests run". Adding MODPY_PYTEST to the
Makefile enables those tests (which run successfully except for one).

cc maintainer

--Kurt

Index: Makefile
===================================================================
RCS file: /cvs/ports/security/py-artifacts/Makefile,v
retrieving revision 1.13
diff -u -p -r1.13 Makefile
--- Makefile 12 Jul 2019 20:49:34 -0000 1.13
+++ Makefile 27 Aug 2019 17:44:31 -0000
@@ -19,6 +19,7 @@ PERMIT_PACKAGE = Yes
MODULES = lang/python
MODPY_PI = Yes
MODPY_SETUPTOOLS = Yes
+MODPY_PYTEST = Yes

FLAVORS = python3
FLAVOR ?=