libgit2: use python 3 for build

Required at build for detection but used only for testing; works fine
on a system without Python 2. If 2 and 3 are installed, 3 is preferred.

OK?


Index: Makefile
===================================================================
RCS file: /cvs/ports/devel/libgit2/libgit2/Makefile,v
retrieving revision 1.44
diff -u -p -r1.44 Makefile
--- Makefile 30 Dec 2019 14:57:50 -0000 1.44
+++ Makefile 31 Mar 2020 21:53:18 -0000
@@ -11,6 +11,7 @@ SHARED_LIBS += git2
MODULES= devel/cmake \
lang/python

+MODPY_VERSION= ${MODPY_DEFAULT_VERSION_3}
MODPY_RUNDEP= No

COMPILER= base-clang ports-gcc base-gcc