test-module-imports: skip on Python < 2.6, since ast is new in 2.6

test-module-imports: skip on Python < 2.6, since ast is new in 2.6

--- a/tests/test-module-imports.t	Sun Nov 17 17:49:48 2013 -0500
+++ b/tests/test-module-imports.t	Sun Nov 17 18:13:55 2013 -0500
@@ -1,3 +1,7 @@
+This code uses the ast module, which was new in 2.6, so we'll skip
+this test on anything earlier.
+  $ python -c 'import sys ; assert sys.version_info >= (2, 6)' || exit 80
+
   $ import_checker="$TESTDIR"/../contrib/import-checker.py
 Run the doctests from the import checker, and make sure
 it's working correctly.