Support python3 as well as python2.

[troycurtisjr]

• Fix configure ac file to use proper print() syntax.
• Add compatibility import for basestring.

[gnomesysadmins]

Thank you for contributing to alacarte!

alacarte uses Bugzilla for code review.

If you have never contributed to GNOME before make sure you have read the
getting started documentation:
http://www.gnome.org/get-involved

Otherwise please visit
https://wiki.gnome.org/Newcomers
and follow the instructions there to upload your change to Bugzilla.

[troycurtisjr]

But Pull Requests are so much faster and easier!

Created bug report and attached patch as instructed: https://bugzilla.gnome.org/show_bug.cgi?id=787490