Add a PY_PILLOW that can be used to choose between graphics/py-pillow

and graphics/py-pillow6

With hat:	portmgr

Mk/Uses/python.mk

@@ -631,8 +631,10 @@ PYNUMPY=	${PYTHON_PKGNAMEPREFIX}numpy>=1.15,1<1.19,1:math/py-numpy@${PY_FLAVOR}
 
 # Common Python modules that can be needed but only for some versions of Python.
 .if ${PYTHON_REL} < 3500
+PY_PILLOW=	${PYTHON_PKGNAMEPREFIX}pillow6>=6.0.0:graphics/py-pillow6@${PY_FLAVOR}
 PY_TYPING=	${PYTHON_PKGNAMEPREFIX}typing>=3.7.4:devel/py-typing@${PY_FLAVOR}
 .else
+PY_PILLOW=	${PYTHON_PKGNAMEPREFIX}pillow>=7.0.0:graphics/py-pillow@${PY_FLAVOR}
 PY_TYPING=
 .endif