Support newer versions of matplotlib (>=3.8)

[siemdejong]

Is your feature request related to a problem? Please describe.
I'm using matplotlibrc files to distribute styles with packages. This is a feature of matplotlib>=3.8 if I'm not mistaken. dabest==2023.03.29 supports matplotlib~=3.6.3. Simply installing a newer matplotlib version (matplotlib==3.9.1.post1) and following https://acclab.github.io/DABEST-python/tutorials/06-plot_aesthetics.html throws

AttributeError: module 'matplotlib.axes' has no attribute '_subplots'

The _subplots attribute has been deprecated in newer matplotlib versions.

Describe the solution you'd like
Support newer versions of matplotlib such that I can use style files in conjunction with dabest.

Describe alternatives you've considered

• Not using dabest
• accept that dabest sometimes throws errors and some functionality is impossible to use with custom style files.

[Jacobluke-]

Hi @siemdejong, thanks for providing solutions for this issue. All the changes has been merged to branch vnbdev.