Add pyproject.toml file

[jdebacker]

One now receives deprecation warnings when building taxcalc from source with pip install -e ..

To avoid this warning, a pyproject.toml should be added to the root directory of the project. See here for more discussion.

[martinholmer]

As of Tax-Calculator 4.3.3, "make package" no longer generates deprecation warnings.

It still may be desirable to replace the setup.py file with a pyproject.toml files as suggested in issue #2803.