Hanging comma on `plots` value in defaults.get_default_plots() causes `sim.plot(to_plot='seir')` to fail

[rbelew]

Describe the bug

Hanging comma here

makes plots become a tuple, and plotting.handle_to_plot() gets confused

To reproduce

For model issues:

import covasim as cv
sim = cv.Sim(pars={}) # Configure the simulation
sim.run() # Steps required to produce the behavior

sim.plot(to_plot='seir')

Expected behavior

Screenshots or outputs

Produces graph attached with comma removed

Platform (please complete the following information):

• Covasim version: 3.1.2
• OS: OSX 10.15.7

Additional context

[cliffckerr]

Fixed -- thanks @rbelew !