gunicorn: convert to Python packaging format + add Python3 variant

gunicorn: convert to Python packaging format + add Python3 variant

And splitting the gunicorn[3] binary/executable away from the
python[3]-gunicorn libraries. This was inspired from Debian packaging.

The gunicorn[3] binaries require the new `python[3]-pkg-resources`
libraries to run, which add ~1.1 MB on the [ram]disk when uncompressed.

For the Python2 variant, the `_gaiohttp.py` is dropped as it fails to
compile, so it would likely be unusable anyway:
```
  File "/usr/lib/python2.7/site-packages/gunicorn/workers/_gaiohttp.py", line 84
    yield from self.wsgi.close()
             ^
  SyntaxError: invalid syntax

```

People around the web recommend this as well:
 https://stackoverflow.com/questions/25611140/syntax-error-installing-gunicorn
 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=803170
 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=803202

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>