[Wheel-builders] Big Sur builds

[Robin Becker]

A user reported a bug when installing reportlab from source on 'Big Sur' (macOS 11). I find I have no way to build 
manylinux wheels (at least on travis with multibuild) for macOS 11. I certainly don't have the hardware (& don't want it).

I don't really understand the OSX software compatibility issues, but travis 
(https://docs.travis-ci.com/user/reference/osx/) doesn't seem to support macos 11 so I guess I will have to wait until 
it and multibuild do before surfing on.
-- 
Robin Becker