Add version requirement to matplotlib

This MR adds a version requirement to matplotlib.

Why was this MR needed?

Dantro references at least one feature that was introduced in matplotlib v3.4 (namely, CenteredNorm).

If the feature is not used, this is usually not an issue. However, I had the problem that the code discovery by the vscode Pylance extension would fail because of import errors, and hence its features could not be used for dantro code. The issue was that my environment solver (conda/mamba) would install matplotlib v3.3. The problem is solved by making the entire code base of dantro importable, which requires at least matplotlib v3.4, but there is currently no specification inside dantro that this is required.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped
  • Reviewed & approved by @

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