Describe the Problem

During verification while migrating to Python 3.8.6, the MatplotlibDeprecationWarning has been noticed. This warning is from within cartopy/mpl/geoaxes.py and corresponds to changes made in Matplotlib version 3.2 and above.

A simple fix to the code in the cycloneplotter wrapper can be accomplished by replacing this:
#Dummy point to add the additional label explaining the labelling of the first
# point in the storm track
plt.scatter(0, 0, zorder=2, marker=None, c='',
label="Date (dd/hhz) is the first " +
"time storm was able to be tracked in model")

with this
#Dummy point to add the additional label explaining the labelling of the first
# point in the storm track
colors = []
plt.scatter(0, 0, zorder=2, marker=None, c=colors,
label="Date (dd/hhz) is the first " +
"time storm was able to be tracked in model")

A plot is still successfully created, but the log file contains this deprecation warning and eventually, when migrating to higher versions of Matplotlib, this will be an issue.

Expected Behavior

Environment

Describe your runtime environment:
*1. Machine: (e.g. HPC name, Linux Workstation, Mac Laptop) Linux
*2. OS: (e.g. RedHat Linux, MacOS) Debian
*3. Software version number(s) Python 3.8.6, cartopy 0.20.2

To Reproduce

Describe the steps to reproduce the behavior:
*1. Run the cyclone plotter use case
*2. Run is successful
*3. Open the log file
*4. Scroll down and find the MatplotlibDeprecationWarning

/d1/personal/mwin/miniconda3/envs/metplus_py386/lib/python3.8/site-packages/cartopy/mpl/geoaxes.py:1546: MatplotlibDeprecationWarning: Using a string of single character colors as a color sequence is deprecated since 3.2 and will be removed two minor releases later. Use an explicit list instead.
result = matplotlib.axes.Axes.scatter(self, *args, **kwargs)

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Funding Source

2792541

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