build: Update lower bound on iminuit to v2.7.0

[matthewfeickert]

Description

This was caught in the Minimum supported dependencies workflow after PR #1845 was merged.

Update lower bound on iminuit to v2.7.0 as it is the 'oldest' SemVer release that has the behavior (as described in the iminiuit v2.7.0 changelog):

Minuit.hesse no longer sets the status of the FunctionMinimum unconditionally to valid if it was invalid before.

Without this fix, the situation can arise where Minuit.hesse() can cause Minuit.valid to be True when it should be False. Specifically in

pyhf/src/pyhf/optimize/opt_minuit.py

Lines 135 to 140 in 2411ed3

	if minimizer.valid:
	# Extra call to hesse() after migrad() is always needed for good error estimates. If you pass a user-provided gradient to MINUIT, convergence is faster.
	minimizer.hesse()
	hess_inv = minimizer.covariance
	corr = hess_inv.correlation()
	unc = minimizer.errors

during

pytest tests/test_pdf.py -k test_pdf_clipping[numpy_minuit]

pyhf/tests/test_pdf.py

Line 1075 in 2411ed3

def test_pdf_clipping(backend):

Also update tests/constraints.txt to use iminuit==2.7.0.

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:

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* Update lower bound on iminuit to v2.7.0 as it is the 'oldest' SemVer release
that has the behavior:
> Minuit.hesse no longer sets the status of the FunctionMinimum unconditionally
> to valid if it was invalid before.

Without this fix, the situation can arise where `Minuit.hesse()` can cause
`Minuit.valid` to be True when it should be False.
* Update tests/constraints.txt to use iminuit==2.7.0.
* Amends PR #1845

[matthewfeickert]

Minimum supported dependencies workflow is passing now. 👍

[codecov]

Codecov Report

Merging #1895 (6d988e0) into master (2411ed3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1895   +/-   ##
=======================================
  Coverage   98.17%   98.17%           
=======================================
  Files          68       68           
  Lines        4331     4331           
  Branches      730      730           
=======================================
  Hits         4252     4252           
  Misses         46       46           
  Partials       33       33           

Flag	Coverage Δ
contrib	26.34% <ø> (ø)
doctest	60.63% <ø> (ø)
unittests-3.10	96.05% <ø> (ø)
unittests-3.7	96.03% <ø> (ø)

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