Revert "move build configuration into `pyproject.toml` (#216)"

[zacharyburnett]

This reverts commit 9af631fa0c0dcf475b0bbcc59545b2c81a2468d8 due to suspected involvement in https://jira.stsci.edu/browse/SPB-1432

It appears that the HCALDP build was explicitly relying on calcos to provide pytest as a dependency, and since calcos's metadata is not parsed correctly for some reason, and the only difference seems to be using PEP621 (setup.py -> pyproject.toml), we are reverting the PEP621 change for now.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (38dbf69) 87.63% compared to head (51a3440) 87.63%.

Additional details and impacted files

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[zacharyburnett]

Hopefully this resolves the issue. After merging this PR we'll have to start the HCALDP job again and then we'll see

[stscirij]

Will try and fix the readthedocs build error in the next PR