build: list direct dependencies explicitly

[alexander-held]

Direct dependencies of cabinetry are now listed explicitly under install_requires. These dependencies are already provided via pyhf, and as such this change does not have an immediate effect. There are advantages to listing them explicitly anyway:

• in case pyhf decided to drop one of the dependencies, cabinetry would break, and this protects against that scenario,
• users can tell more easily which other libraries cabinetry makes direct use of,
• the dependency tree is more correct, as visualized e.g. by pipdeptree.

The slightly more verbose setup.cfg seems like a negligible problem in comparison.

This partially reverts #168.

* explicitly require all direct dependencies of cabinetry via install_requires

[codecov]

Codecov Report

Merging #261 (39ee9ea) into master (e565ec8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #261   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           18        18           
  Lines         1742      1742           
  Branches       269       269           
=========================================
  Hits          1742      1742           

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