Upstream changes:
0.18
[#103] Add new mime types:
repr_plotly1
repr_vega4
repr_vegalite2
repr_vdom
0.17
Fix tests on Windows
0.16
[#96] Fix displaying lists with mixed named and unnamed items
[IRkernel/IRkernel#588] Fixed in #98 and #99 independently
Each R package should include ../../math/R/Makefile.extension, which also
defines MASTER_SITES. Consequently, it is redundant for the individual
packages to do the same. Package-specific definitions also prevent
redefining MASTER_SITES in a single common place.