matplotlib versions older than 3.5 crash with cabinetry 0.5.0

[alexander-held]

When using matplotlib versions older than 3.5, the layout keyword argument usage in plt.figure / plt.subplots crashes (used in cabinetry.visualize), as this was only introduced in 3.5. To fix this, we should either raise the minimum required matplotlib version or refactor the implementation to avoid the use of this keyword argument.

(reported by @kratsg, thank you!)