MNT: transition matplotlib code to set_layout_engine

[nkeim]

Several tests with latest dependencies now fail with PendingDeprecationWarning('The set_tight_layout function will be deprecated in a future version. Use set_layout_engine instead.')

It looks like at this stage, we should avoid changing the code if possible since set_layout_engine was introduced only about a year ago. But now that the warning has done its job, we should change the tests to ignore it.