Apply fontsize to both x- and y-axis tick labels
 - [x] closes #15108
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Pls add tests to check font size, using ``_check_ticks_props``. Sure - I added a test for both `TestDataFramePlots` and `TestDataFrameGroupByPlots`. Let me know if only one test (the former?) is sufficient though. lgtm. pls add whats new note.

@TomAugspurger ? OK - I hope this was the right place to add the note. Thanks!