configure.ac: migrate from python2 to python3

Fortunatelly we don't have any python2 specific code, so just bump.

Change-Id: Iaab94a1e896b78dc8699256e326edc165b0ed922
Related: OS#5950
diff --git a/configure.ac b/configure.ac
index 65ebd59..484d689 100644
--- a/configure.ac
+++ b/configure.ac
@@ -117,9 +117,9 @@
 				[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