feat: Add type hints for tensor manager

[kratsg]

Pull Request Description

See #1284.

This PR adds typing for src/pyhf/tensor/manager.py (and also adds some typing for the src/pyhf/events.py to make sure the decorators used are forwarding their signatures).

Additionally provides some minor fix on top of #1940 to get the NumPy backend more properly typed according to the expanded (stricter) TensorBackend Protocol.

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 type hints to the tensor manager.
* Add type hints to some of the events management code for the decorators used.
* Expand the TensorBackend protocol and fix up the NumPy backend type hints to align properly.
   - Amends PR #1940.

[codecov]

Codecov Report

Base: 98.26% // Head: 98.26% // No change to project coverage 👍

Coverage data is based on head (b12c9dd) compared to base (b12c9dd).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1963   +/-   ##
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  Coverage   98.26%   98.26%           
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  Files          68       68           
  Lines        4430     4430           
  Branches      728      728           
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  Hits         4353     4353           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	27.04% <0.00%> (ø)
doctest	60.88% <0.00%> (ø)
unittests-3.10	96.18% <0.00%> (ø)
unittests-3.7	96.16% <0.00%> (ø)
unittests-3.8	96.20% <0.00%> (ø)
unittests-3.9	96.23% <0.00%> (ø)

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[kratsg]

@matthewfeickert docs look not so bad here: https://pyhf--1963.org.readthedocs.build/en/1963/_generated/pyhf.get_backend.html#pyhf.get_backend

[matthewfeickert]

LGTM. 👍 Thanks @kratsg!