refactor: Use TensorViewer in SciPy optimizer, instead of explicit constraints

[kratsg]

Pull Request Description

Part of pulling out/refactoring #951. We need to split out the portion of rewriting scipy to use TensorViewer first, so we know the changes in the tests, before doing a larger refactor.

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• Tests are passing
• "WIP" removed from the title of the pull request
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* Rewrite the scipy_optimizer to use TensorViewer and drop constraints

[codecov]

Codecov Report

Merging #970 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #970      +/-   ##
==========================================
+ Coverage   96.48%   96.50%   +0.02%     
==========================================
  Files          57       57              
  Lines        3217     3236      +19     
  Branches      441      442       +1     
==========================================
+ Hits         3104     3123      +19     
  Misses         72       72              
  Partials       41       41              

Flag	Coverage Δ
#unittests	96.50% <100.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
src/pyhf/infer/__init__.py	100.00% <ø> (ø)
src/pyhf/infer/mle.py	100.00% <ø> (ø)
src/pyhf/infer/test_statistics.py	100.00% <ø> (ø)
src/pyhf/__init__.py	98.00% <100.00%> (+0.08%)	⬆️
src/pyhf/optimize/opt_scipy.py	100.00% <100.00%> (ø)

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[matthewfeickert]

Overall looking very nice, and I appreciate the work that factoring this out took. Just have a few questions and then we need @lukasheinrich's review.

[kratsg]

Closing in favor of #951 which is preserving all of the functionality instead of removing the option to not-stitch entirely.