ci: do not test Python 2.7 with Windows
ci: do not test Python 2.7 with Windows

The Python 2.7 tests fail in Windows with vivisect because the Windows
filesystem encoding is not UTF-8. This shouldn't be a problem when using
capa as the given filename most likely uses the same encoding, but we
force UTF-8 in our tests. As we are planing to remove Python 2 support
is not wortwhile to invest time in making this test working. Instead,
test Python 2.7 only in Ubuntu.

mandiant/capa,mandiant/capa