Change packaging backend to use pyproject.toml with flit

[mwtoews]

An alternative proposal to #66 is to use flit for packaging. Building is similar:

$ python3 -m pip install --upgrade build
...
$ python3 -m build
* Creating venv isolated environment...
* Installing packages in isolated environment... (flit_core >=2,<4)
* Getting dependencies for sdist...
* Building sdist...
* Building wheel from sdist
* Creating venv isolated environment...
* Installing packages in isolated environment... (flit_core >=2,<4)
* Getting dependencies for wheel...
* Building wheel...
Successfully built affine-2.3.0.tar.gz and affine-2.3.0-py2.py3-none-any.whl

the wheel installs fine with Python 2.7, if that still matters.

[sgillies]

Hey @mwtoews I'm game for either this or #66. What are the pros and cons of each? Which is your favorite and why?

[mwtoews]

Not sure if I have a favourite yet, but leaning towards flit (see why use flit doc). It was recently adopted as a PyPA Project, which boosts its credibility even further. I don't see any downsides.

[mwtoews]

This PR is updated to resolve conflicts. Also, I can re-add a setup.py shim, if that's an expected file by someone.

[sgillies]

For some reason, GitHub wouldn't let me resolve conflict here, so I did it manually. Done in 9fed202. Thanks @mwtoews !