mypy: add types-six to the list of installed packages (#24207)
mypy: add types-six to the list of installed packages (#24207)

See https://mypy-lang.blogspot.com/2021/05/the-upcoming-switch-to-modular-typeshed.html
for a broader explanation.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack