py38-cairo-1.20.0 conflicts with py27-cairo-1.18.2nb1?

["John D. Baker" <jdbaker%consolidated.net@localhost>]

I've seen this error reported in a couple of places, but was claimed
to be a symptom of some other issue and not logged to "pkgsrc-users@"
or a PR of its own.

With pkgsrc-HEAD as the freeze for 2020Q4 approaches, installing
py27-cairo-1.18.2nb1 (graphics/py-cairo118) is claimed to conflict
with py37-cairo-1.20.0 (graphics/py-cairo):

===> Installing binary package of py27-cairo-1.18.2nb1
pkg_add: Installed package `py38-cairo-1.20.0' conflicts with `py27-cairo-[0-9]*' when trying to install `py27-cairo-1.18.2nb1'.
pkg_add: 1 package addition failed

A similar error occurs if py27-cairo-1.18.2nb1 is already installed and one
tries to install py38-cairo-1.20.0.

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