requirements: update matplotlib requirement from <3.7.6 to <3.9.4

[dependabot]

Updates the requirements on matplotlib to permit the latest version.

Release notes

Sourced from matplotlib's releases.

REL: 3.9.3

This is the third bugfix release of the 3.9.x series.

This release contains several bug-fixes and adjustments:

• Fix axline with extremely small slopes
• Fix axline with non-linear axis scales
• Fix minimumSizeHint with Qt backend
• Fix config directory usage when it's behind a symlink
• Fix draggable legend when blitting is enabled
• Fix high CPU utilization in the macosx backend
• Fix multiple hatch edgecolors passed to contourf
• Improve compatibility with pytest 8.2.0

Commits

• 3ac0aea REL: 3.9.3
• 3f7adbd Merge branch 'v3.9.2-doc' into v3.9.x
• 4ca8d68 DOC: Create release notes for 3.9.3
• 0cabfe2 Merge pull request #29195 from meeseeksmachine/auto-backport-of-pr-29191-on-v...
• 562d458 Backport PR #29191: ci: Simplify 3.13t test setup
• 0586854 Merge pull request #29176 from meeseeksmachine/auto-backport-of-pr-29148-on-v...
• 84f2ae2 Merge pull request #29178 from meeseeksmachine/auto-backport-of-pr-29163-on-v...
• dd57772 Backport PR #29163: ci: Remove outdated pkg-config package on macOS
• c4bfd54 Backport PR #29148: Don't fail on equal-but-differently-named cmaps in qt fig...
• d71ff49 Backport PR #29153: Bump codecov/codecov-action from 4 to 5 in the actions gr...
• Additional commits viewable in compare view

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[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.11):

diff -r docs-base/ModulatoryProjection.html docs-head/ModulatoryProjection.html
272c272
< <a class="reference internal" href="Port.html"><span class="doc">Port</span></a> are listed in the Port’s <a class="reference internal" href="Mechanism.html#id7" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.mod_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">mod_afferents</span></code></a> attribute.</p>
---
> <a class="reference internal" href="Port.html"><span class="doc">Port</span></a> are listed in the Port’s <a class="reference internal" href="ParameterPort.html#psyneulink.core.components.ports.parameterport.ParameterPort.mod_afferents" title="psyneulink.core.components.ports.parameterport.ParameterPort.mod_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">mod_afferents</span></code></a> attribute.</p>
diff -r docs-base/Port.html docs-head/Port.html
616c616
< <code class="xref any docutils literal notranslate"><span class="pre">PathWayProjections</span></code> (listed in its <a class="reference internal" href="Mechanism.html#id6" title="psyneulink.core.components.mechanisms.mechanism.Mechanism_Base.path_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">path_afferents</span></code></a> attribute) as the variable for its
---
> <code class="xref any docutils literal notranslate"><span class="pre">PathWayProjections</span></code> (listed in its <a class="reference internal" href="#psyneulink.core.components.ports.port.Port_Base.path_afferents" title="psyneulink.core.components.ports.port.Port_Base.path_afferents"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">path_afferents</span></code></a> attribute) as the variable for its
diff -r docs-base/TransferFunctions.html docs-head/TransferFunctions.html
2199c2199
< <p><a class="reference internal" href="#psyneulink.core.components.functions.transferfunctions.Exponential.bounds" title="psyneulink.core.components.functions.transferfunctions.Exponential.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to
---
> <p><a class="reference internal" href="OptimizationFunctions.html#psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds" title="psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to

...

See CI logs for the full diff.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.11):

diff -r docs-base/TransferFunctions.html docs-head/TransferFunctions.html
2199c2199
< <p><a class="reference internal" href="#psyneulink.core.components.functions.transferfunctions.Exponential.bounds" title="psyneulink.core.components.functions.transferfunctions.Exponential.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to
---
> <p><a class="reference internal" href="OptimizationFunctions.html#psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds" title="psyneulink.core.components.functions.optimizationfunctions.GradientOptimization.bounds"><code class="xref any py py-attr docutils literal notranslate"><span class="pre">bounds</span></code></a> – specifies the lower and upper limits of the result;  if there are none, the attribute is set to

...

See CI logs for the full diff.

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.11):

No differences!

...

See CI logs for the full diff.