move build configuration into pyproject.toml

[zacharyburnett]

setuptools now supports the [project] table, which is defined by PEP621.

Additionally, setuptools now supports its own entry in pyproject.toml called [tool.setuptools] (pypa/setuptools#1688, https://setuptools.pypa.io/en/latest/userguide/pyproject_config.html#setuptools-specific-configuration); however, it comes with the following disclaimer:

Support for declaring configurations not standardized by PEP 621 (i.e. the [tool.setuptools] table), is still in beta stage and might change in future releases.

It is likely that this functionality with setuptools will leave beta stage upon the release of Python 3.11, with its native implementation of tomli

Given this, we can attempt to consolidate the build configuration into a single pyproject.toml file that can possibly be read by other build systems in the future.

[zacharyburnett]

removing reliance on relic with this PR would also solve this issue: spacetelescope/stenv#78

[sean-lockwood]

@zacharyburnett -
The RTD site was rebuilt around the time this PR was merged. It looks like the docs built without autodoc, as most of the function descriptions are now blank.

E.g.:
https://stistools.readthedocs.io/en/latest/inttag.html

[zacharyburnett]

The RTD site was rebuilt around the time this PR was merged. It looks like the docs built without autodoc, as most of the function descriptions are now blank.

That is strange; they populate on HTML built locally on my machine. I will look into this issue.