tests: use absolute_import for heredoctest.py

tests/heredoctest.py

@@ -1,3 +1,5 b''
+from __future__ import absolute_import
+
 import sys
 
 globalvars = {}

tests/test-check-py3-compat.t

@@ -146,7 +146,6 b''
   tests/filterpyflakes.py requires print_function
   tests/generate-working-copy-states.py requires print_function
   tests/get-with-headers.py requires print_function
-  tests/heredoctest.py not using absolute_import
   tests/heredoctest.py requires print_function
   tests/hghave.py not using absolute_import
   tests/hgweberror.py not using absolute_import