Add MODPY_TEST (should be MODPY_PYTEST) as poisoned entry

I'd like to add MODPY_TEST as a poisoned entry to mk.conf.template. I
used to make this typo a lot. Things look like they work right, but
pytest isn't getting used at that point, just the usual setuptools
testing mechanism.

ok?

--Kurt

Index: mk.conf.template
===================================================================
RCS file: /cvs/ports/infrastructure/templates/mk.conf.template,v
retrieving revision 1.27
diff -u -p -r1.27 mk.conf.template
--- mk.conf.template 6 Nov 2017 20:43:22 -0000 1.27
+++ mk.conf.template 30 Nov 2019 00:37:52 -0000
@@ -50,6 +50,7 @@
.poison WKRDIST
.poison WKRINST
.poison WKRSRC
+.poison MODPY_TEST

# not a misspelling, but wrong, BUILD_DEPENDS isn't subpackaged
.poison BUILD_DEPENDS-main