Upgrade Dockerfile to python3

[adrian-marza-mambu]

The container can't be built with python2 anymore, so we might as well switch to the python3 packages fully.

The error appears at docker build:

docker build -t review-rot:latest .
...
Step 10/12 : RUN pip install --upgrade pip setuptools && python setup.py install
 ---> Running in 80f8e409519f
Collecting pip
  Downloading https://files.pythonhosted.org/packages/54/eb/4a3642e971f404d69d4f6fa3885559d67562801b99d7592487f1ecc4e017/pip-20.3.3-py2.py3-none-any.whl (1.5MB)
Collecting setuptools
  Downloading https://files.pythonhosted.org/packages/82/2f/ef7afd98530d07c89deffec833b4b1a91a27a5db6d9f1a216599f5f0316e/setuptools-51.1.2.tar.gz (2.1MB)
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "setuptools/__init__.py", line 16, in <module>
        import setuptools.version
      File "setuptools/version.py", line 1, in <module>
        import pkg_resources
      File "pkg_resources/__init__.py", line 1365
        raise SyntaxError(e) from e
                                ^
    SyntaxError: invalid syntax

After this fix docker-build works:

docker build -t review-rot:latest .
...
Successfully tagged review-rot:latest

docker run --rm -it review-rot:latest bash
review-rot --help
...

[adrian-marza-mambu]

@pbortlov @danrodrig @ralphbean PTAL if you get a chance

[danrodrig]

@adrian-marza-mambu I didn't look into this project since quite a while and I never used the container image. However, I did a quick check. I built and run the image with Podman and I was able to check some reviews, LGTM.