Some are producing ever-so-slightly nondeterministic results, and others demonstrate a Python 2 vs. 3 issue. While #5307 got most of these stuff, these were punted on because obvious solutions haven't yet been found.

• test_axes_grid1:test_twin_axes_empty_and_removed
• test_axes:test_boxplot
• test_axes:test_boxplot_rc_parameters
• test_axes:test_specgram_freqs
• test_axes:test_specgram_noise
• test_axes:test_specgram_magnitude_freqs
• test_axes:test_hist_steplog