[python-package] migrate test_plotting.py to pytest

[thomasjpfan]

Towards #3732

[jameslamb]

Thanks so much for all your help! I just left one small suggestion about linting.

I reviewed with https://www.textcompare.org/index.html?id=600a657520516c001761fc4c.

I like the use of module-scoped features to replicated what we were doing in setUp(), thank you.

[jameslamb]

Suggested change

	assert (ax1.get_title() ==
	'Histogram for feature name {}'.format(gbm1.booster_.feature_name()[27]))
	title = 'Histogram for feature name {}'.format(gbm1.booster_.feature_name()[27])
	assert ax1.get_title() == title

I think this change would address the issue from linting ():

Linting Python code
./tests/python_package_test/test_plotting.py:91:29: W504 line break after binary operator

[StrikerRUS]

LGTM!
Thank you!

[github-actions]

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