fix: Ensure that setting upper limit works for toms748 scan

[kratsg]

Pull Request Description

• resolves pyhf.infer.intervals.upper_limits.upper_limit ignores the set confidence level when scan=None #2563

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

* Pass 'level' kwarg from the upper_limits.upper_limit through to toms748 scan.
* Add test to ensure that a different level being set is actually providing
  different limits.

[matthewfeickert]

Thanks @kratsg! I'll fixup the CI and review this this week.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (7de53ab) to head (92930f3).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2564   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files          65       65           
  Lines        4289     4289           
  Branches      461      461           
=======================================
  Hits         4215     4215           
  Misses         46       46           
  Partials       28       28           

Flag	Coverage Δ
contrib	98.15% <ø> (ø)
doctest	98.27% <ø> (ø)
unittests-3.10	96.45% <ø> (ø)
unittests-3.11	96.45% <ø> (ø)
unittests-3.12	96.45% <ø> (ø)
unittests-3.13	96.45% <ø> (ø)
unittests-3.9	96.52% <ø> (ø)

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[matthewfeickert]

Thanks very much for this fix @kratsg, and apologies @GiacomoXT (and @kratsg) that this is something that got forgotten for a long time.