fix pcolormesh grid deprecation warning with matplotlib 3.5

[sbailey]

This PR avoids a deprecation warning when using matplotlib 3.5:

pytest py/desutil/test
...
test/test_plots.py::TestPlots::test_plot_sky_binned
  /global/common/software/desi/users/sjbailey/desiutil/py/desiutil/plots.py:718: MatplotlibDeprecationWarning: Auto-removal of grids by pcolor() and pcolormesh() is deprecated since 3.5 and will be removed two minor releases later; please call grid(False) first.
    mesh = ax.pcolormesh(grid_ra, grid_dec,

However, I haven't looked into what is actually happening with turning the grid off and on, so I'd appreciate a check by someone who knows what a real-world output plot is supposed to look like with this code. @weaverba137 @dkirkby ?

This is housecleaning for a 21.12 software release for Fuji, but is low priority in the big picture.

[coveralls]

Coverage increased (+0.01%) to 69.156% when pulling f04985c on pcolormesh_grid into db67f80 on master.

[weaverba137]

@sbailey, there is an example notebook that shows what these plots are supposed to look like. You may want to run through that, but I can do that too if you don't have time.

[sbailey]

@weaverba137 yes, it would be helpful if you could run the notebook and do the checks. Please also check with a custom kernel that tests with matplotlib 3.5; from the command line one version of that is:

base=/global/common/software/desi/users/sjbailey
module use $base/desiconda2/modulefiles

module load desiconda/2.0 
module load desiutil/master
module load desitree/0.5.0
module load QuasarNP/0.1.2
module load specex/0.8.3
module load specter/master
module load desimodel/master
module load desitarget/master
module load specsim/master
module load desispec/master
module load desisim/master
module load fiberassign/master
module load desisurvey/master
module load surveysim/master
module load redrock/master
module load redrock-templates/master
module load prospect/master
module load desimeter/master
module load simqso/master
module load speclite/master

(you probably don't need more of those modules past desiutil, but I'm listing what modules are available in that test installation just in case...) If you have a different setup for testing with alternate package versions, that is fine too.

[weaverba137]

I've run through the entire notebook using your matplotlib install. All the plots looked identical.

[sbailey]

Thanks for the cross check; I'll merge now and update changes.rst.