configure.ac: Require python3 for ext_tests

The python scripts already use #!/usr/bin/env python3 so it was pure
coincidence that the tests are working.

Change-Id: I96ac31e7862fe102e5baee0c2e25458ff0451a50
diff --git a/configure.ac b/configure.ac
index 12af7e6..346e020 100644
--- a/configure.ac
+++ b/configure.ac
@@ -172,9 +172,9 @@
 				[Include the VTY/CTRL tests in make check [default=no]]),
 		[enable_ext_tests="$enableval"],[enable_ext_tests="no"])
 if test "x$enable_ext_tests" = "xyes" ; then
-	AC_CHECK_PROG(PYTHON2_AVAIL,python2,yes)
-	 if test "x$PYTHON2_AVAIL" != "xyes" ; then
-		AC_MSG_ERROR([Please install python2 to run the VTY/CTRL tests.])
+	AC_CHECK_PROG(PYTHON3_AVAIL,python3,yes)
+	 if test "x$PYTHON3_AVAIL" != "xyes" ; then
+		AC_MSG_ERROR([Please install python3 to run the VTY/CTRL tests.])
 	fi
 	AC_CHECK_PROG(OSMOTESTEXT_CHECK,osmotestvty.py,yes)
 	 if test "x$OSMOTESTEXT_CHECK" != "xyes" ; then