windows: pin Windows 10 SDK version

Windows CI stopped working several days ago because GitHub Actions
upgraded the Windows Driver Kit to the Windows 11 version. This was
reported against CPython in https://bugs.python.org/issue45220.
Although they incorrectly triaged it as a Windows 11 SDK upgrade:
the GitHub Runner changes I bisected this to
(actions/runner-images@win19/20210907.4...win19/20210914.2#diff-816219d3c9f30fcc1c40f88ca826c1662bf99eded781c1b419dd2dfb577ae227)
do not indicate a Windows SDK upgrade but a WDK upgrade.

Author: indygreg

cpython-windows/build.py

@@ -1322,6 +1322,10 @@ def run_msbuild(
         "/property:IncludeTests=%s" % ("false" if static else "true"),
         "/property:OverrideVersion=%s" % python_version,
         "/property:IncludeCTypes=true",
+        # We pin the Windows 10 SDK version to make builds more deterministic.
+        # This can also work around known incompatibilities with the Windows 11
+        # SDK as of at least CPython 3.9.7.
+        "/property:DefaultWindowsSDKVersion=10.0.20348.0",
     ]
 
     exec_and_log(args, str(pcbuild_path), os.environ)