test: Use scikit-hep-testdata to provide probability models for regression tests

[matthewfeickert]

Description

With scikit-hep/scikit-hep-testdata#67, the sbottom probability models that are used for regression tests are added to scikit-hep-testdata. This provides stability in testing that is independent of HEPData's availability and doesn't abuse HEPData either as we currently ping them a lot with our test suite.

This also simplifies the tests a bit by removing the need for a fixture to provide the tarfile archive.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add scikit-hep-testdata to the 'test' extra
* Use scikit-hep-testdata to provide the file path to the tar.gz that
contains the sbottom probability models used for regression tests
(pyhf-ins1748602-probability-models.tar.gz)
   - File added in release scikit-hep-testdata v0.4.11
* Refactor get_json_from_tarfile pytest fixture to open the tarfile archive
* Remove sbottom_likelihoods_download pytest fixture as no longer needed

[codecov]

Codecov Report

Merging #1711 (13fbe14) into master (34dd45c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1711   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files          64       64           
  Lines        4267     4267           
  Branches      593      593           
=======================================
  Hits         4187     4187           
  Misses         46       46           
  Partials       34       34           

Flag	Coverage Δ
contrib	26.27% <ø> (ø)
doctest	60.83% <ø> (ø)
unittests	96.17% <ø> (ø)

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