feat: Type hints

[kratsg]

Description

Resolves #1284

See:

• https://www.python.org/dev/peps/pep-0484/
• https://www.python.org/dev/peps/pep-0544/
• https://pypi.org/project/typing-extensions/
• https://github.com/scikit-hep/hist/issues/64
• https://github.com/Princeton-Penn-Vents/princeton-penn-flowmeter/blob/2f71f613aafc04ea604f3ff7ed7be1b99a40b4cc/nurse/qt.py#L92-L131

typing-extensions is used for python 3.6, 3.7 for certain things such as Protocol classes.

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

• [ ] Summarize commit messages into a comprehensive review of the PR

[codecov[bot]]

Codecov Report

Merging #948 (bcef1cf) into master (025ffe9) will decrease coverage by 1.01%. The diff coverage is 55.10%.

@@            Coverage Diff             @@
##           master     #948      +/-   ##
==========================================
- Coverage   97.65%   96.63%   -1.02%     
==========================================
  Files          63       64       +1     
  Lines        4006     4104      +98     
  Branches      565      566       +1     
==========================================
+ Hits         3912     3966      +54     
- Misses         55       98      +43     
- Partials       39       40       +1     

Flag	Coverage Δ
contrib	25.09% <8.16%> (-0.42%)	:arrow_down:
unittests	96.41% <55.10%> (-1.02%)	:arrow_down:

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Impacted Files	Coverage Δ
src/pyhf/typing.py	53.48% <53.48%> (ø)
src/pyhf/tensor/jax_backend.py	96.42% <66.66%> (-0.66%)	:arrow_down:
src/pyhf/tensor/numpy_backend.py	97.76% <66.66%> (-0.72%)	:arrow_down:
src/pyhf/tensor/pytorch_backend.py	97.67% <66.66%> (-0.74%)	:arrow_down:
src/pyhf/tensor/tensorflow_backend.py	96.59% <66.66%> (-0.63%)	:arrow_down:

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[alexander-held]

#1472 provides some motivation for typing of the tensor return values from the pyhf API.