facet_col in imshow assumes xarray coordinates are integers

[Karl-Krauth]

Minimal example:

arr = xr.DataArray(np.random.randn(2, 3, 3), dims=("string", "1", "2"), coords={"string": ["A", "B"]})
px.imshow(arr, facet_col="string")

Which prints the following error message:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[54], line 2
      1 arr = xr.DataArray(np.random.randn(2, 3, 3), dims=("string", "1", "2"), coords={"string": ["A", "B"]})
----> 2 px.imshow(arr, facet_col="string")

File ~/.bin/mambaforge/lib/python3.10/site-packages/plotly/express/_imshow.py:544, in imshow(img, zmin, zmax, origin, labels, x, y, animation_frame, facet_col, facet_col_wrap, facet_col_spacing, facet_row_spacing, color_continuous_scale, color_continuous_midpoint, range_color, title, template, width, height, aspect, contrast_rescaling, binary_string, binary_backend, binary_compression_level, binary_format, text_auto)
    540 if facet_col is not None:
    541     slice_label = (
    542         "facet_col" if labels.get("facet_col") is None else labels["facet_col"]
    543     )
--> 544     col_labels = ["%s=%d" % (slice_label, i) for i in facet_slices]
    545 fig = init_figure(args, "xy", [], nrows, ncols, col_labels, [])
    546 for attr_name in ["height", "width"]:

File ~/.bin/mambaforge/lib/python3.10/site-packages/plotly/express/_imshow.py:544, in <listcomp>(.0)
    540 if facet_col is not None:
    541     slice_label = (
    542         "facet_col" if labels.get("facet_col") is None else labels["facet_col"]
    543     )
--> 544     col_labels = ["%s=%d" % (slice_label, i) for i in facet_slices]
    545 fig = init_figure(args, "xy", [], nrows, ncols, col_labels, [])
    546 for attr_name in ["height", "width"]:

TypeError: %d format: a real number is required, not numpy.str_