configure: Check for version of pytest

Addresses ElementsProject#486 and takes into account @rustyrussell 's
comment about PYTEST defined in the environment which
always overrides without checking the version.

configure

@@ -66,6 +66,7 @@ find_pytest()
     PYTEST_BINS="pytest-3 pytest3 pytest py.test"
     for p in $PYTEST_BINS; do
 	if [ "$(which $p)" != "" ] ; then
+	    "$p" --version 2>&1 | grep -q "python3" || continue
 	    echo "$p"
 	    return
 	fi
@@ -117,8 +118,6 @@ rm -f $CONFIG_VAR_FILE.$$
 $CONFIGURATOR --autotools-style --var-file=$CONFIG_VAR_FILE.$$ --header-file=$CONFIG_HEADER --configurator-cc="$CONFIGURATOR_CC" "$CC" $CWARNFLAGS $CDEBUGFLAGS
 mv $CONFIG_VAR_FILE.$$ $CONFIG_VAR_FILE
 
-find_pytest
-
 add_var PREFIX "$PREFIX"
 add_var CC "$CC"
 add_var CONFIGURATOR_CC "$CONFIGURATOR_CC"