AppVeyor: don't download huge pillow-depends.zip

[hugovk]

On AppVeyor, we're downloading a zip of the entire contents of https://github.com/python-pillow/pillow-depends/ mirror which is 851 MB and on my machine takes 6 minutes. We don't use all the files so don't need to download them all.

extract_dep of winbuild/build_prepare.py first checks if there's a local copy, and if there's not, downloads it from the upstream source.

Instead of downloading the whole mirror, I've modified extract_dep: if there's not a local copy, it first tries to download from our mirror. If that doesn't succeed, it tried the upstream source.

Also add some type hints and capitalise constants.

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Also, curl has started failing to download with:

curl -fsSL -o pillow-depends.zip https://github.com/python-pillow/pillow-depends/archive/main.zip
curl: (35) schannel: next InitializeSecurityContext failed: Unknown error (0x80092013) - The revocation function was unable to check revocation because the revocation server was offline.
Command exited with code 35

https://ci.appveyor.com/project/Python-pillow/pillow/builds/48061132/job/i2a2jg124obvmoud

Adding --ssl-no-revoke fixes it:

• https://stackoverflow.com/a/56486639/724176
• https://stackoverflow.com/a/63996289/724176

[radarhere]

AppVeyor has started passing again - https://ci.appveyor.com/project/Python-pillow/pillow/builds/48066083. I think that means adding --ssl-no-revoke is no longer necessary?

[hugovk]

Good, I've dropped the --ssl-no-revoke commit.