add win-arm64 support

[niyas-sait]

This patch enables building lxml package for win/arm64 platforms.

The change expects win/arm64 libraries to be available in repository currently used for getting x86 and x64 dependencies. The packages are not yet available in the repository but a PR has been raised to add arm64 libraries.

Unofficial release of win/arm64 libraries from PR are available here

Test Result

TESTED VERSION: 4.6.3
    Python:           sys.version_info(major=3, minor=10, micro=0, releaselevel='candidate', serial=1)
    lxml.etree:       (4, 6, 3, 0)
    libxml used:      (2, 9, 5)
    libxml compiled:  (2, 9, 5)
    libxslt used:     (1, 1, 30)
    libxslt compiled: (1, 1, 30)
    FS encoding:      utf-8
    Default encoding: utf-8
    Max Unicode:      1114111

----------------------------------------------------------------------
Ran 1937 tests in 7.474s

OK

[niyas-sait]

@scoder This is probably not quite ready yet considering the library dependency is not yet available in the libxml2-win-binaries repository. But would be nice to hear your thoughts on how to resolve the dependency etc. to get this sorted for win/arm64 targets.

[scoder]

It doesn't hurt to merge it already.

Regarding Windows library building, I actually don't know how they were previously built. Having a build script for Github Actions would certainly be very much appreciated.

[niyas-sait]

Thanks, @scoder.

As the external dependencies for windows were pulled from libxml2-win-binaries, I've created a PR to extend the appveyor build scripts and related files in libxml2-win-binaries to add win-arm64 packages.

Unfortunately, the maintainer of libxml2-win-binaries doesn't have time to maintain the repository and he left a comment for you to look for an owner for that repository. Could you suggest what would be the best way to resolve this issue?

I've also created a full library release through appveyor here which includes packages for all supported platforms including win-arm64.

[bastimeyer]

This diff breaks the arch reference in the first if-block and breaks the py27 builds on appveyor:

• https://ci.appveyor.com/project/scoder/lxml/builds/41249423/job/vnu6gmaa3u0f4by7#L49
• https://ci.appveyor.com/project/scoder/lxml/builds/41249423/job/173d08dq3bbx6mef#L57

Not sure how relevant this is considering you're currently trying to sort out the dependency builds in the remote repository, but just wanted to let you know in case you haven't noticed already.

[niyas-sait]

Yes I think the statements in lines 41 and 42 should be moved after line 48

[niyas-sait]

Opened #331 to fix it

[scoder]

Thanks, yeah, I noticed this, too, when I was trying to set up an automated wheel build.

[scoder]

(Sorry for breaking your original PR)