* allow VTK UnstructuredGrid to be empty

* add test

Github has discontinued 3.6 support on macOS11.
See https://github.com/actions/virtual-environments/issues/4060

* CI Tests on MacoS

* add $

* make silo optional

* don't test silo on MacOS

* float128 -> longdouble

* make dtype kinds more generic

https://github.com/inducer/modepy/pull/38#issuecomment-846472970

Run flake8 in Github CI on target Py version

CI/README: master->main

Small fixes for XDMF

Seems to be an XDMF2 thing and is not present in the source anymore.

Fix libsilo download URL for Github CI

XDMF support

Examples doc ci

See merge request inducer/pyvisfile!6