feat: customizable POI for significance

[alexander-held]

This is an extension of #348 to also support poi_name in fit.significance. This allows to specify the parameter for which the significance calculation is performed.

The implementation follows #364, and as such also requires at least the pyhf RC 4 for v0.7 that was added there.

* added ability to set POI via poi_name keyword argument in fit.significance

[codecov]

Codecov Report

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

Coverage data is based on head (6a96853) compared to base (0a92ee0).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #365   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files           23        23           
  Lines         2013      2019    +6     
  Branches       330       331    +1     
=========================================
+ Hits          2013      2019    +6     

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

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