Using Visual Studio 2017 to build the current master branch of Python (something I'm trying for the first time in about two years - maybe I'm missing something obvious!), with the x64 target, under both the Release and Debug builds I get a Python that can't even start to run the test suite.  Here's output from the Release build - the Debug build output is nearly identical.  The problem does not appear with the Win32 target.  Of course I'm doing this on a 64-bit box (under Win 10 Pro):

C:\Code\Python\PCbuild>rt -q -x64

C:\Code\Python\PCbuild>"C:\Code\Python\PCbuild\amd64\python.exe"  -u -Wd -E -bb -m test
Traceback (most recent call last):
  File "C:\Code\Python\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Code\Python\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Code\Python\lib\test\__main__.py", line 1, in <module>
    from test.libregrtest import main
  File "C:\Code\Python\lib\test\libregrtest\__init__.py", line 4, in <module>
    from test.libregrtest.cmdline import _parse_args, RESOURCE_NAMES, ALL_RESOURCES
  File "C:\Code\Python\lib\test\libregrtest\cmdline.py", line 4, in <module>
    from test import support
  File "C:\Code\Python\lib\test\support\__init__.py", line 6, in <module>
    import asyncio.events
  File "C:\Code\Python\lib\asyncio\__init__.py", line 8, in <module>
    from .base_events import *
  File "C:\Code\Python\lib\asyncio\base_events.py", line 39, in <module>
    from . import events
  File "C:\Code\Python\lib\asyncio\events.py", line 14, in <module>
    import contextvars
  File "C:\Code\Python\lib\contextvars.py", line 1, in <module>
    from _contextvars import Context, ContextVar, Token, copy_context
ModuleNotFoundError: No module named '_contextvars'


_contextvarsmodule.c is in the list of Modules under the pythoncore project, and it appears to compile without incident when the Solution is built.

The executable appears to be fine in other respects I checked; e.g.,

C:\Code\Python\PCbuild>amd64\python.exe
Python 3.8.0a0 (heads/master:24f6846920, Sep 14 2018, 18:55:33) [MSC v.1915 64 bit (AMD64)] on win32

Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> from collections import Counter
>>> Counter("abab")
Counter({'a': 2, 'b': 2})
>>> import _contextvars
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named '_contextvars'

Clue?