mpl 3.3+ deprecation warning

[bjlittle]

📰 Custom Issue

PR #3762 unpinned matplotlib<3.3.

However, the tests are now generating the following matplotlib deprecation warning, which should be resolved before a hard traceback is encountered from matplotlib>=3.5 onwards:

.../iris/lib/iris/plot.py:252: MatplotlibDeprecationWarning: 
The IndexFormatter class was deprecated in Matplotlib 3.3 and will be removed two minor releases later.
  formatter = mpl_ticker.IndexFormatter(ticks)

This issue is based on SciTools/iris latest master commit 13e8b2c.

For example, the above deprecation warning can be recreated by running the following iris unit test:

> python lib/iris/tests/unit/quickplot/test_scatter.py

[trexfeathers]

Closing since discussion is taking place in duplicate #3848