AppVeyor: build Python in 64-bit mode

[vstinner]

Previously, Python was built in 32-bit mode and so issues specific to
64-bit mode like compiler warnings could be missed.

[vstinner]

Example of a recent warning missed by AppVeyor: #8359

[serhiy-storchaka]

Couldn't issues specific to 32-bit mode be missed?

[vstinner]

We have 32 and 64 bit Windows buildbots. I expect that Python has more 64 bit users on Windows. I recall more issues on 64 bit Windows, compiler warnings are the most common.

[serhiy-storchaka]

I'm not a Windows expert, but these changes LGTM.

[zware]

My goal for AppVeyor is just speed (that's why it builds in Release configuration rather than Debug), so I'd rather it use whichever is faster between 32- and 64-bit. If they're effectively the same, then switching to 64-bit is fine with me.

FTR, VSTS already does both 32- and 64-bit builds, and it appears from those logs that 64-bit is generally about 2 minutes slower than 32-bit.

[vstinner]

FTR, VSTS already does both 32- and 64-bit builds, and it appears from those logs that 64-bit is generally about 2 minutes slower than 32-bit.

I looked at AppVeyor tests timing:

• 32-bit: 4 min 44 sec
• 64-bit: 4 min 38 sec

So there is no significant difference.

--

I looked at the "VSTS: Windows" logs on this PR: I confirm that the 64 bits build is 2 min slower than the 32 bits build. No idea why. But this PR only changes AppVeyor configuration.

[vstinner]

I switched AppVeyor to 64-bit on the master branch. My use case is to catch compiler warnings, and usually I only fix compiler warnings in the master branch. So I'm not sure about backporting this change to 2.7, 3.6 and 3.7 branches. What do you think?