Fix broken import in mplexporter tests

[mgorny]

Fixes#5253

Code PR

• I have read through the contributing notes and understand the structure of the package. In particular, if my PR modifies code of plotly.graph_objects, my modifications concern the code generator and not the generated files.
• I have added tests or modified existing tests.
• For a new feature, I have added documentation examples (please see the doc checklist as well).
• I have added a CHANGELOG entry if changing anything substantial.
• For a new feature or a change in behavior, I have updated the relevant docstrings in the code.

[emilykl]

I'm fine with merging this, but it's worth mentioning that the mplexporter tests aren't currently running in the CI (which is how this was missed in the first place).

There are also two failing mplexporter tests currently:

FAILED plotly/matplotlylib/mplexporter/tests/test_basic.py::test_legend_dots - AssertionError: assert '3' == '2'
FAILED plotly/matplotlylib/mplexporter/tests/test_utils.py::test_linestyle - AssertionError: 

I've opened #5360 to track.

[mgorny]

Thank you!

[emilykl]

Thank you for the fix @mgorny !! 🚀