chore: Migrate from diana-hep GitHub org to scikit-hep

[matthewfeickert]

Description

Migrate from the DIANA/HEP GitHub org to the Scikit-HEP org

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

* Replace all instances of diana-hep with scikit-hep

[kratsg]

We lost:

• CodeFactor
• LGTM analysis
• Semantic Pull Request

[matthewfeickert]

We lost:

• CodeFactor
• LGTM analysis
• Semantic Pull Request

CodeFactor and LGTM are back. They just needed time to update.

• 
• 

What's the issue with Semantic Pull Request?

[codecov]

Codecov Report

Merging #643 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #643   +/-   ##
=======================================
  Coverage   94.93%   94.93%           
=======================================
  Files          42       42           
  Lines        2547     2547           
  Branches      345      345           
=======================================
  Hits         2418     2418           
  Misses         85       85           
  Partials       44       44

Flag	Coverage Δ
#unittests	94.93% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/pyhf/utils.py	97.11% <100%> (ø)	⬆️

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