Warnings when used with pandas 2.1.0

[schloerke]

From has2k1/plotnine#713

With pandas 2.1.0, using plotnine results in a number of warnings from is_categorical_dtype.

For example:

from plotnine import ggplot, geom_point, aes, stat_smooth, facet_wrap
from plotnine.data import mtcars

ggplot(mtcars, aes("wt", "mpg", color="factor(gear)")) + geom_point()
#> /opt/homebrew/lib/python3.11/site-packages/plotnine/utils.py:1183: FutureWarning: is_categorical_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, CategoricalDtype) instead
#> /opt/homebrew/lib/python3.11/site-packages/plotnine/utils.py:289: FutureWarning: is_categorical_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, CategoricalDtype) instead
#> /opt/homebrew/lib/python3.11/site-packages/mizani/scale.py:210: FutureWarning: is_categorical_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, CategoricalDtype) instead
#> /opt/homebrew/lib/python3.11/site-packages/mizani/scale.py:223: FutureWarning: is_categorical_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, CategoricalDtype) instead