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.

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.

[codecov]

Codecov Report

Base: 0.00% // Head: 0.00% // No change to project coverage 👍

Coverage data is based on head (4576e39) compared to base (1c14205).
Patch has no changes to coverable lines.

Additional details and impacted files

@@          Coverage Diff           @@
##           master     #56   +/-   ##
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  Coverage    0.00%   0.00%           
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  Files          25      25           
  Lines        3060    3060           
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  Misses       3060    3060           

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[zacharyburnett]

I was able to get the tests to run through correctly by removing usage of tox, similar to in spacetelescope/stcal#96

[zacharyburnett]

@WilliamJamieson I didn't realize that stpipe doesn't have a RTD site, I can make a PR to add that so we can get rid of the docs job in github actions