Resolve various warnings during build-and-test

Created by: alurban

This PR addresses several runtime warnings that arise during build-and-test:

• Do not duplicate the 'cmap' parameter between the mappable and ax.colorbar() (see matplotlib release notes)
• Log-scaled plot axes have deprecated kwargs (see matplotlib release notes)
• The numpy.ndarray.tostring() method is deprecated as of 1.19, and its replacement tobytes has been available since 1.9 (see numpy documentation)
• Warnings about numpy.ufunc size changes are benign (they arise from downstream packages built against earlier numpy versions) and can be ignored, see this stackoverflow post
• CLI plot products carry a redundancy in grid kwargs when the grid is turned off
• CLI coherencegrams carry an ambiguous truth value of an astropy.units.Quantity

Other warnings arise from an advertised change to the default spectral averaging method (addressed in #1282) and deprecated behavior related to gwosc and uproot, which can be handled in a separate PR. There are also upstream warnings arising from glue.ligolw running python-3.8, presumably because the package was built with an earlier version of python.

Note, this updates the Matplotlib version requirement to 3.3.0, numpy to 1.15.0 (for consistency with Matplotlib), and python-ligo-lw to 1.7.0.

This fixes #1289 (closed).