Add Python 3 support.

* Add Python 3 support.
* d/control: Split the gunicorn package into separate binary packages
  for the Python 2 and 3 library, the Python 2 and 3 command line
  scripts, and common files.  Update build dependencies for Python 3.
* d/compat: Bump to version 8.
* d/patches/0001-drop-supplemental-groups.patch: Adapt for Python 3.
* d/rules:
  - Use --buildsystem=pybuild and dh_python3.
  - Update overrides for proper package installation.
* d/gunicorn.examples -> gunicorn-common.examples
* d/gunicorn.init -> gunicorn-common.init
* d/gunicorn.install -> gunicorn-common.install
* d/gunicorn3.manpages: Added.
* wrap-and-sort
Comment out DH_VERBOSE.
d/control:

* d/control:
  - Split the gunicorn package into separate binary packages for the
    Python 2 and 3 library, the Python 2 and 3 command line scripts, and
    common files.
  - Update build dependencies for Python 3.
  - Build the gunicorn-debian.8 manpage with help2man.
* d/gunicorn{3,-common}.manpages: Added.
Add a Closes: