[3.9] bpo-40826: PyOS_InterruptOccurred() requires GIL (GH-20578) (GH-20618)

* bpo-40826: Add _Py_EnsureTstateNotNULL() macro (GH-20571)

Add _Py_EnsureTstateNotNULL(tstate) macro: call Py_FatalError() if
tstate is NULL, the error message contains the current function name.

(cherry picked from commit 3026cad59b87751a9215111776cac8e819458fce)

* bpo-40826: PyOS_InterruptOccurred() requires GIL (GH-20578)

PyOS_InterruptOccurred() now fails with a fatal error if it is called
with the GIL released.

(cherry picked from commit cbe129692293251e7fbcea9ff0d822824d90c140)
diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index 5c7526a..aad9239 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -67,7 +67,10 @@
         self.assertTrue(err.rstrip().startswith(
                          b'Fatal Python error: '
                          b'PyThreadState_Get: '
-                         b'current thread state is NULL (released GIL?)'))
+                         b'the function must be called with the GIL held, '
+                         b'but the GIL is released '
+                         b'(the current Python thread state is NULL)'),
+                        err)
 
     def test_memoryview_from_NULL_pointer(self):
         self.assertRaises(ValueError, _testcapi.make_memoryview_from_NULL_pointer)