Add basic pyproject.toml

[aktech]

The setup.py and package dependencies are quite complicated. It would be non-trivial to replace all with pyproject.toml in one PR. This is one of many PRs to surgically remove setup.py and use pyproject.toml.

Since pyproject.toml is a static configuration file in contrast to dynamic setup.py, we're trying to use both at the moment until we fully replace everything in setup.py.

We're currently building three packages in this project:

• tfx
• ml-pipelines-sdk
• tfx-dev (for development, which is a union of the above two)

I have created explicit pyproject.toml files for the three types of builds we're doing. There might be some duplication in these files, but now is not the time to extract them, until we fully replace setup.py and old build machinery (including installation of specific components via optional dependencies instead of environment variables).

[peytondmurray]

Need a double check on the pytest-runner build time dependency.