Backport PR #4273 on branch 1.12.x (test: diffmap plot coverage)

[meeseeksmachine]

Backport PR #4273: test: diffmap plot coverage

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
1983	1	1982	1038

View the top 1 failed test(s) by shortest run time

tests/plotting/legacy/test_plotting_utils.py::tests.plotting.legacy.test_plotting_utils

Stack Traces | 0s run time

#x1B[31mImportError while importing test module '.../plotting/legacy/test_plotting_utils.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
#x1B[1m#x1B[.../plotting/legacy/test_plotting_utils.py#x1B[0m:13: in <module>
    #x1B[0m#x1B[94mfrom#x1B[39;49;00m#x1B[90m #x1B[39;49;00m#x1B[04m#x1B[96mscanpy#x1B[39;49;00m#x1B[04m#x1B[96m.#x1B[39;49;00m#x1B[04m#x1B[96mplotting#x1B[39;49;00m#x1B[04m#x1B[96m.#x1B[39;49;00m#x1B[04m#x1B[96mlegacy#x1B[39;49;00m#x1B[04m#x1B[96m.#x1B[39;49;00m#x1B[04m#x1B[96m_tools#x1B[39;49;00m#x1B[04m#x1B[96m.#x1B[39;49;00m#x1B[04m#x1B[96mscatterplots#x1B[39;49;00m#x1B[90m #x1B[39;49;00m#x1B[94mimport#x1B[39;49;00m _get_palette#x1B[90m#x1B[39;49;00m
#x1B[1m#x1B[31mE   ModuleNotFoundError: No module named 'scanpy.plotting.legacy'#x1B[0m#x1B[0m

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