Update matplotlib requirement from ~=3.8 to ~=3.9

Updates the requirements on [matplotlib](https://github.com/matplotlib/matplotlib) to permit the latest version.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.8.0...v3.9.0)

---
updated-dependencies:
- dependency-name: matplotlib
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,6 +1,6 @@
 sunpy~=5.1
 astropy~=6.0
 numpy~=1.26
-matplotlib~=3.8
+matplotlib~=3.9
 networkx~=3.3
 ndcube~=2.2

requirements_dev.txt

@@ -1,7 +1,7 @@
 sunpy~=5.1
 astropy~=6.0
 numpy~=1.26
-matplotlib~=3.8
+matplotlib~=3.9
 networkx~=3.3
 ndcube~=2.2