build: Update SciPy lower bound to v1.5.4

[matthewfeickert]

Description

• SciPy v1.5.4 is the first SciPy release to have Python 3.9 wheels.
  • c.f. https://pypi.org/project/scipy/1.5.4/#files
• Update scipy to v1.5.4 in tests/constraints.txt to enforce the lower bound for the minimum supported dependencies tests.

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

* SciPy v1.5.4 is the first SciPy release to have Python 3.9 wheels.
   - c.f. https://pypi.org/project/scipy/1.5.4/#files
* Update scipy to v1.5.4 in tests/constraints.txt to enforce the
  lower bound for the minimum supported dependencies tests.

[matthewfeickert]

Minimum supported dependencies workflow check passing on my fork:

[Copilot]

Pull Request Overview

This PR updates the minimum required scipy version from 1.5.2 to 1.5.4 across dependency specifications.

Key changes:

• Updated scipy minimum version constraint from 1.5.2 to 1.5.4 in both pyproject.toml and tests/constraints.txt

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/constraints.txt	Updated pinned scipy version for testing from 1.5.2 to 1.5.4
pyproject.toml	Updated minimum scipy version requirement from >=1.5.2 to >=1.5.4 in project dependencies

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (0f52681) to head (858b363).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2633   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files          65       65           
  Lines        4190     4190           
  Branches      453      453           
=======================================
  Hits         4116     4116           
  Misses         45       45           
  Partials       29       29           

Flag	Coverage Δ
contrib	98.11% <ø> (ø)
doctest	98.23% <ø> (ø)
unittests-3.10	96.42% <ø> (ø)
unittests-3.11	96.42% <ø> (ø)
unittests-3.12	96.42% <ø> (ø)
unittests-3.13	96.42% <ø> (ø)
unittests-3.9	96.46% <ø> (ø)

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