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
BLD: test_perf tweaks
TST/BUG: disallow bs4==4.2.0 and skip assoc tests
BUG: Display the legend when calling plot with kind='density'. GH4216
Check if a plotting DataFrame with kind='kde' has a legend. GH4216
Updated release note about GH4216.

BUG: legend was not displayed in `DataFrame.plot(kind='kde')`.
Merge pull request #4232 from cpcloud/read-html-warnings

CLN/TST: clean up and raise on bs4 version and no tables
Merge pull request #4221 from garaud/plot_density_legend

BUG: Display the legend when calling plot with kind='density'. GH4216