Using theme_matplotlib() fails with `'object' object has no attribute 'startswith'`

Hi, all. On my machine theme_matplotlib() consistently fails. Here's an example:

from pandas import DataFrame
from plotnine import *

theme_set(theme_matplotlib())

PLOT = (
 ggplot(DataFrame([{"X":0,"Y":0},{"X":1,"Y":2}])) +
 aes('X', 'Y') +
 geom_point()
);
PLOT.save('test.png')

This is what I get when I run it:

Traceback (most recent call last):
  File "test.py", line 11, in <module>
    PLOT.save('test.png')
  File ".../site-packages/plotnine/ggplot.py", line 664, in save
    sv = self.save_helper(
  File ".../site-packages/plotnine/ggplot.py", line 613, in save_helper
    figure = self.draw(show=False)
  File ".../site-packages/plotnine/ggplot.py", line 272, in draw
    with plot_context(self, show=show):
  File ".../site-packages/plotnine/_utils/context.py", line 48, in __enter__
    self.rc_context.__enter__()
  File ".../contextlib.py", line 135, in __enter__
    return next(self.gen)
  File ".../site-packages/matplotlib/__init__.py", line 1188, in rc_context
    rcParams.update(rc)
  File ".../_collections_abc.py", line 994, in update
    self[key] = other[key]
  File ".../site-packages/matplotlib/__init__.py", line 733, in __setitem__
    cval = self.validate[key](val)
  File ".../site-packages/matplotlib/rcsetup.py", line 252, in validate_backend
    s if s is _auto_backend_sentinel or s.startswith("module://")
AttributeError: 'object' object has no attribute 'startswith'

This is with Python 3.10.5, matplotlib 3.8.4, and 0.13.6.

Is there anything I can do to make this work?

has2k1 / plotnine

Using theme_matplotlib() fails with `'object' object has no attribute 'startswith'` #784