make Mtest.py aware of the (not that) new conditional "HAVE_PYTHON3"
make Mtest.py aware of the (not that) new conditional "HAVE_PYTHON3"

no guarantees that any (future) tests requiring Python3 will actually work,
since we are not yet aware of the necessary differences in setting PYTHONPATH
for Python(2) vs. Python3 ...

zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb,zyzyis/monetdb