Bug summary
Passing in a one element list splits it up as if it was string input, when it should treat it as 1 axes.

Code for reproduction

fig, axd = plt.subplot_mosaic(['triangle'])

Actual outcome

Expected outcome

Matplotlib version

• Operating system:
• Matplotlib version (import matplotlib; print(matplotlib.__version__)):
• Matplotlib backend (print(matplotlib.get_backend())):
• Python version:
• Jupyter version (if applicable):
• Other libraries: