bpo-40268: Rename _PyInterpreterState_GET_UNSAFE() (GH-19509)

Rename _PyInterpreterState_GET_UNSAFE() to _PyInterpreterState_GET()
for consistency with _PyThreadState_GET() and to have a shorter name
(help to fit into 80 columns).

Add also "assert(tstate != NULL);" to the function.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 492988e..95b023d 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -994,7 +994,7 @@
 {
     assert(errors != NULL && errors != Py_None);
 
-    PyInterpreterState *interp = _PyInterpreterState_GET_UNSAFE();
+    PyInterpreterState *interp = _PyInterpreterState_GET();
 #ifndef Py_DEBUG
     /* In release mode, only check in development mode (-X dev) */
     if (!_PyInterpreterState_GetConfig(interp)->dev_mode) {