Use PyThreadState_GetDict, avoid Python internals

Works around problems caused by exposing Py internals (Py_BUILD_CORE).

- Build error with GCC, Py3.6 & OpenMP
- Linking error on MSVC

source/blender/python/generic/py_capi_utils.c

@@ -29,12 +29,6 @@
  * BLI_string_utf8() for unicode conversion.
  */
 
-/* TODO, resolve linking errors on win32 */
-#ifndef _WIN32
-/* needed for Py3.6+ to access Py_PyThreadState_Current */
-#define Py_BUILD_CORE
-#endif
-
 #include <Python.h>
 #include <frameobject.h>
 
@@ -671,8 +665,8 @@ void PyC_SetHomePath(const char *py_path_bundle)
 
 bool PyC_IsInterpreterActive(void)
 {
-	/* expanded PyThreadState_GET which won't throw an exception */
+	/* instead of PyThreadState_Get, which calls Py_FatalError */
-	return (((PyThreadState *)_Py_atomic_load_relaxed(&_PyThreadState_Current)) != NULL);
+	return (PyThreadState_GetDict() != NULL);
 }
 
 /* Would be nice if python had this built in