Broken imports in matplotlib tests in 6.2.0

[mgorny]

Since the 6.2.0 release, pytest fails to collect matplot tests:

========================================================= test session starts =========================================================
platform linux -- Python 3.13.5, pytest-8.4.1, pluggy-1.6.0
rootdir: /tmp/plotly.py
configfile: pyproject.toml
plugins: anyio-4.9.0, typeguard-4.4.4
collected 3383 items / 1 error                                                                                                        

=============================================================== ERRORS ================================================================
________________________________ ERROR collecting plotly/matplotlylib/mplexporter/tests/test_utils.py _________________________________
ImportError while importing test module '/tmp/plotly.py/plotly/matplotlylib/mplexporter/tests/test_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.13/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
plotly/matplotlylib/mplexporter/tests/test_utils.py:2: in <module>
    from . import plt
E   ImportError: cannot import name 'plt' from 'plotly.matplotlylib.mplexporter.tests' (/tmp/plotly.py/plotly/matplotlylib/mplexporter/tests/__init__.py)
========================================================== warnings summary ===========================================================
plotly/conftest.py:4
  /tmp/plotly.py/plotly/conftest.py:4: PytestRemovedIn9Warning:
  
  The (path: py.path.local) argument is deprecated, please use (collection_path: pathlib.Path)
  see https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path

tests/test_io/test_to_from_plotly_json.py:112
  /tmp/plotly.py/tests/test_io/test_to_from_plotly_json.py:112: UserWarning:
  
  no explicit representation of timezones available for np.datetime64

.venv/lib/python3.13/site-packages/pdfrw/objects/pdfstring.py:120
  /tmp/plotly.py/.venv/lib/python3.13/site-packages/pdfrw/objects/pdfstring.py:120: SyntaxWarning:
  
  invalid escape sequence '\('

.venv/lib/python3.13/site-packages/pdfrw/objects/pdfstring.py:375
  /tmp/plotly.py/.venv/lib/python3.13/site-packages/pdfrw/objects/pdfstring.py:375: SyntaxWarning:
  
  invalid escape sequence '\['

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
======================================================= short test summary info =======================================================
ERROR plotly/matplotlylib/mplexporter/tests/test_utils.py
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
==================================================== 4 warnings, 1 error in 8.27s =====================================================

This seems to be caused by eb95abc that removed the plt import from plotly/matplotlylib/mplexporter/tests/__init__.py.

To reproduce:

pip install -e .[dev_optional] matplotlib
pytest

CC @gvwilson

[mgorny]

Sorry, meant 6.2.0.