PEtab validation should not require matplotlib

[dweindl]

... but currently it does:

Checking visualization table...
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.9.17/x64/bin/petablint", line 8, in <module>
    sys.exit(main())
  File "/opt/hostedtoolcache/Python/3.9.17/x64/lib/python3.9/site-packages/petab/petablint.py", line 132, in main
    ret = petab.lint.lint_problem(problem)
  File "/opt/hostedtoolcache/Python/3.9.17/x64/lib/python3.9/site-packages/petab/lint.py", line 887, in lint_problem
    from petab.visualize.lint import validate_visualization_df
  File "/opt/hostedtoolcache/Python/3.9.17/x64/lib/python3.9/site-packages/petab/visualize/__init__.py", line 10, in <module>
    from .plot_data_and_simulation import (
  File "/opt/hostedtoolcache/Python/3.9.17/x64/lib/python3.9/site-packages/petab/visualize/plot_data_and_simulation.py", line 6, in <module>
    import matplotlib.pyplot as plt
ModuleNotFoundError: No module named 'matplotlib'