configure: Prefer pytest over pytest3

When looking for a pytest executable we should be looking for the ones
that `pip` installs in a virtualenv (`pytest` and `py.test`) before we
look for the ones that `apt` installs system-wide (`pytest3` and
`py.test3`) because these may not be part of the virtualenv that all
other packages are installed in.

configure

@@ -56,7 +56,7 @@ default_cwarnflags()
 
 default_pytest()
 {
-    PYTEST_BINS="pytest-3 pytest3 pytest py.test"
+    PYTEST_BINS="pytest py.test pytest3 pytest-3"
     for p in $PYTEST_BINS; do
 	if [ "$(which $p)" != "" ] ; then
 	    "$p" --version 2>&1 | grep -q "python3" || continue