This is a continuation of msg269741 from #26624, summarizing how to resolve this message.

 C:\WINDOWS\SYSTEM32\ucrtbased.dll is version 10.0.10240.16384
 WARN: ucrtbased contains known issues. Please update Visual Studio or the Windows SDK.  See: http://bugs.python.org/issue26624

F:\Python\dev\36\PCbuild\python.vcxproj(91,5): warning MSB3073: The command ""F:\Python\dev\36\PCBuild\win32\python_d.exe" "F:\Python\dev\36\PC\validate_ucrtbase.py" ucrtbased" exited with code 1.

Merely updating Visual Studio is not enough.  One must update the Windows SDK either directly or while updating Visual Studio by making sure that the new SDK is requested under Features.

[X] Windows and Web Development
    [X] Universal Windows App Development Tools
        [X] Tools (1.4.1) and Windows 10 SDK (10.0.14393)

(Versions and even the menu may change in the future.)

The best way to update Visual Studio is probably to uninstall it first via  Control Panel / Programs and Features / Microsoft Visual Studio Community 2015 (or whichever version you have).

I did not.  After downloading the current installer
  vs_community__9c4d694197656e43997d069a0ead2003.exe
I just ran without uninstalling.  It automatically installs in the current vs2015 directory,  Instead of a clean install, it updated and ended with the same 'bad hash' message as reported in msg269741.

I then went to Programs and Features and chose 'Repair'.  The first time it failed because it did not like the existing 
  Microsoft Visual C++ 2015 Redistributable (x86 or x64)
files being present.  With those removed, and the SDK feature checked, Repair worked and Python builds without the warning.

The current SDK is not listed as 'Microsoft Windows SDK...' but as
  Windows Software Development Kit - Windows 10.0.14393

Steve: my suggestion is that "Please update Visual Studio or the Windows SDK." be changed to "Please update the Windows SDK, possibly by updating Visual Studio with the Windows SDK feature selected."  Also change the issue number from 26624 to whatever this one gets.