BUG: raise on invalid colormap for older mpl
DOC: release notes
CLN: correct format string
Merge pull request #4231 from cpcloud/matplotlib-invalid-colomap-test

BUG: raise on invalid colormap for older mpl
BUG: define __getslice__ in terms of __getitem__
CLN: clean up some whitespace
CLN/TST: clean up and add test for slice with names
TST: add tests for slicing with names
DOC: release notes