fix: set POI for limit calculations via dedicated pyhf API

[alexander-held]

#348 added the possibility to specify a custom POI via a kwarg in fit.limit. This was done by setting model.config.poi_name and model.config.poi_index directly, which is no longer possible after scikit-hep/pyhf#1972 (see scikit-hep/pyhf#1984). scikit-hep/pyhf#1985 adds support for None POI values in model.config.set_poi, which allows that API to be used instead. This PR switches to this dedicated API to support temporarily changing the POI, which requires an update of the minimum pyhf version to 0.7.0rc4.

* use pyhf model.config.set_poi API to support temporary POI changes in limit calculations

[codecov]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (c19b5f4) compared to base (a8ca551).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #364   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           23        23           
  Lines         2018      2013    -5     
  Branches       330       330           
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- Hits          2018      2013    -5     

Impacted Files	Coverage Δ
src/cabinetry/fit/__init__.py	100.00% <100.00%> (ø)

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