It looks like _png was removed in 3.3.0:

https://github.com/matplotlib/matplotlib/pull/15193

On Tue, Jul 28, 2020 at 5:19 PM Steiner, Andrew (Andrew W. Steiner) via yt-users <yt-users@python.org> wrote:
Hello all,

       After installing yt 3.6.0 and matplotlib 3.3.0
on 10.15.6 I'm getting the error below after running
'import yt' regarding matplotlib._png. Any ideas?

Thank you,
Andrew

     import yt
   File "/usr/local/lib/python3.8/site-packages/yt/__init__.py", line
63, in <module>
     from yt.fields.api import \
   File "/usr/local/lib/python3.8/site-packages/yt/fields/api.py", line
19, in <module>
     from . import angular_momentum
   File
"/usr/local/lib/python3.8/site-packages/yt/fields/angular_momentum.py",
line 26, in <module>
     from .vector_operations import \
   File
"/usr/local/lib/python3.8/site-packages/yt/fields/vector_operations.py",
line 32, in <module>
     from yt.geometry.geometry_handler import \
   File
"/usr/local/lib/python3.8/site-packages/yt/geometry/geometry_handler.py",
line 29, in <module>
     from yt.utilities.parallel_tools.parallel_analysis_interface import \
   File
"/usr/local/lib/python3.8/site-packages/yt/utilities/parallel_tools/parallel_analysis_interface.py",
line 30, in <module>
     from yt.data_objects.image_array import ImageArray
   File
"/usr/local/lib/python3.8/site-packages/yt/data_objects/image_array.py",
line 18, in <module>
     from yt.visualization.image_writer import write_bitmap, write_image
   File
"/usr/local/lib/python3.8/site-packages/yt/visualization/image_writer.py",
line 29, in <module>
     import yt.utilities.png_writer as pw
   File
"/usr/local/lib/python3.8/site-packages/yt/utilities/png_writer.py",
line 13, in <module>
     import matplotlib._png as _png
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