bpo-33166: Change os.cpu_count to return active (real) processors (GH-15949)

(cherry picked from commit aa92927)

Co-authored-by: Steve Dower <steve.dower@python.org>

Author: miss-islington

Misc/NEWS.d/next/Windows/2019-09-11-14-42-04.bpo-36634.8Un8ih.rst

@@ -0,0 +1,2 @@
+:func:`os.cpu_count` now returns active processors rather than maximum
+processors.

Modules/posixmodule.c

@@ -12176,23 +12176,9 @@ os_cpu_count_impl(PyObject *module)
 {
     int ncpu = 0;
 #ifdef MS_WINDOWS
-    /* Vista is supported and the GetMaximumProcessorCount API is Win7+
-       Need to fallback to Vista behavior if this call isn't present */
-    HINSTANCE hKernel32;
-    static DWORD(CALLBACK *_GetMaximumProcessorCount)(WORD) = NULL;
-    Py_BEGIN_ALLOW_THREADS
-    hKernel32 = GetModuleHandleW(L"KERNEL32");
-    *(FARPROC*)&_GetMaximumProcessorCount = GetProcAddress(hKernel32,
-        "GetMaximumProcessorCount");
-    Py_END_ALLOW_THREADS
-    if (_GetMaximumProcessorCount != NULL) {
-        ncpu = _GetMaximumProcessorCount(ALL_PROCESSOR_GROUPS);
-    }
-    else {
-        SYSTEM_INFO sysinfo;
-        GetSystemInfo(&sysinfo);
-        ncpu = sysinfo.dwNumberOfProcessors;
-    }
+    /* Declare prototype here to avoid pulling in all of the Win7 APIs in 3.8 */
+    DWORD WINAPI GetActiveProcessorCount(WORD group);
+    ncpu = GetActiveProcessorCount(ALL_PROCESSOR_GROUPS);
 #elif defined(__hpux)
     ncpu = mpctl(MPC_GETNUMSPUS, NULL, NULL);
 #elif defined(HAVE_SYSCONF) && defined(_SC_NPROCESSORS_ONLN)