Drop dependency on Python "future" package

The `future` Python package isn't included in Debian or Fedora, so those downstreams (among others?) will have to package it before they can ship the new VIPS Python bindings.  However, VIPS' use of `future` seems to be limited: it just replaces a few builtins with versions that provide Python 3 semantics.  Since none of the call sites for those builtins appear to depend on Python 3 semantics, it seems the `future` dependency can be dropped.

Tested with `test_python.sh` with `future` uninstalled.  The only failure is `test_icc` on Python 3, which is also failing in master.