bpo-39947: Use _PyInterpreterState_GET_UNSAFE() (GH-18978)

Replace _PyInterpreterState_Get() function call with
_PyInterpreterState_GET_UNSAFE() macro which is more efficient but
don't check if tstate or interp is NULL.

_Py_GetConfigsAsDict() now uses _PyThreadState_GET().
diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index f4ded2e..76bc48d 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -95,7 +95,7 @@
     PyCompilerFlags local_flags = _PyCompilerFlags_INIT;
     int nomem_count = 0;
 #ifdef Py_REF_DEBUG
-    int show_ref_count = _PyInterpreterState_Get()->config.show_ref_count;
+    int show_ref_count = _PyInterpreterState_GET_UNSAFE()->config.show_ref_count;
 #endif
 
     filename = PyUnicode_DecodeFSDefault(filename_str);
@@ -346,7 +346,7 @@
     filename_obj = PyUnicode_DecodeFSDefault(filename);
     if (filename_obj == NULL)
         return -1;
-    PyInterpreterState *interp = _PyInterpreterState_Get();
+    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
     bootstrap = PyObject_GetAttrString(interp->importlib,
                                        "_bootstrap_external");
     if (bootstrap != NULL) {
@@ -1117,7 +1117,7 @@
 
     /* Set globals['__builtins__'] if it doesn't exist */
     if (globals != NULL && PyDict_GetItemString(globals, "__builtins__") == NULL) {
-        PyInterpreterState *interp = _PyInterpreterState_Get();
+        PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
         if (PyDict_SetItemString(globals, "__builtins__", interp->builtins) < 0) {
             return NULL;
         }