Supply the _subprocess module under 7.1.  I'm not sure what the status
of this should be on non-WIN32 Windows variants.
diff --git a/PCbuild/pythoncore.vcproj b/PCbuild/pythoncore.vcproj
index 59498b3..59bddb3 100644
--- a/PCbuild/pythoncore.vcproj
+++ b/PCbuild/pythoncore.vcproj
@@ -364,6 +364,9 @@
 			RelativePath="..\Modules\_sre.c">
 		</File>
 		<File
+			RelativePath="..\Pc\_subprocess.c">
+		</File>
+		<File
 			RelativePath="..\Modules\_weakref.c">
 			<FileConfiguration
 				Name="Release|Win32">