Always depend on gnureadline if setupext.check_for_readline() fails

The inconsistency (asymmetry) of unconditionally adding gnureadline as a
dependency of binary distributions can cause issues like reported at
https://github.com/paylogic/pip-accel/issues/34
Depend on gnureadline when building binary wheel or setupext.check_for_readline() fails

This is a follow up to 4d9f6f00a4f0798e41700c22a839eddff0881692 based on
the discussion in pull request #7047 where we seem to have found a middle
ground that should make everyone happy :-)
Merge pull request #7047 from xolox/setup-gnureadline-fix

Only force gnureadline in setup.py with bdist_wheel

instead of all bdists