appveyor: Add libxcvt build dep

Install libxcvt build dep on appveyor.

Explicitly install python3.8 lxml to ensure it matches python version
installed (to workaround issues with Cygwin installer).

Drop explicit configuration of hal and udev, as meson.build now knows to
turn those off for Cygwin.
5 jobs for windows-meson-fixes in 3 minutes and 31 seconds (queued for 21 seconds)
latest
Status Name Job ID Coverage
  Docker Image
passed debian-buster #15455819

00:00:16

 
  Build And Test
passed meson #15455820

00:03:15

passed meson-noglamor #15455821

00:01:36

passed mingw-cross-build #15455822

00:00:46

 
  External
passed continuous-integration/appveyor/branch