Rewrite dict literal for files in tests/plotting/test_boxplot_method.py
- [ ] xref #38138
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Rewrites as dictionary literals for the following files:
- pandas/tests/plotting/test_boxplot_method.py
