tests: ensure demandimport test uses absolute_import

tests/test-check-py3-compat.t

@@ -20,7 +20,6 b''
   contrib/python-zstandard/tests/test_module_attributes.py not using absolute_import
   contrib/python-zstandard/tests/test_train_dictionary.py not using absolute_import
   setup.py not using absolute_import
-  tests/test-demandimport.py not using absolute_import
 
 #if py3exe
   $ testrepohg files 'set:(**.py) - grep(pygments)' \

tests/test-demandimport.py

@@ -1,4 +1,4 b''
-from __future__ import print_function
+from __future__ import absolute_import, print_function
 
 from mercurial import demandimport
 demandimport.enable()