bpo-38076 Clear the interpreter state only after clearing module globals (GH-18039)

Currently, during runtime destruction, `_PyImport_Cleanup` is clearing the interpreter state before clearing out the modules themselves. This leads to a segfault on modules that rely on the module state to clear themselves up.

For example, let's take the small snippet added in the issue by @DinoV :
```
import _struct

class C:
    def __init__(self):
        self.pack = _struct.pack
    def __del__(self):
        self.pack('I', -42)

_struct.x = C()
```

The module `_struct` uses the module state to run `pack`. Therefore, the module state has to be alive until after the module has been cleared out to successfully run `C.__del__`. This happens at line 606, when `_PyImport_Cleanup` calls `_PyModule_Clear`. In fact, the loop that calls `_PyModule_Clear` has in its comments: 

> Now, if there are any modules left alive, clear their globals to minimize potential leaks.  All C extension modules actually end up here, since they are kept alive in the interpreter state.

That means that we can't clear the module state (which is used by C Extensions) before we run that loop.

Moving `_PyInterpreterState_ClearModules` until after it, fixes the segfault in the code snippet.

Finally, this updates a test in `io` to correctly assert the error that it now throws (since it now finds the io module state). The test that uses this is: `test_create_at_shutdown_without_encoding`. Given this test is now working is a proof that the module state now stays alive even when `__del__` is called at module destruction time. Thus, I didn't add a new tests for this.


https://bugs.python.org/issue38076

Lib/test/test_struct.py

@@ -7,6 +7,7 @@ import struct
 import sys
 
 from test import support
+from test.support.script_helper import assert_python_ok
 
 ISBIGENDIAN = sys.byteorder == "big"
 
@@ -652,6 +653,23 @@ class StructTest(unittest.TestCase):
         s2 = struct.Struct(s.format.encode())
         self.assertEqual(s2.format, s.format)
 
+    def test_struct_cleans_up_at_runtime_shutdown(self):
+        code = """if 1:
+            import struct
+
+            class C:
+                def __init__(self):
+                    self.pack = struct.pack
+                def __del__(self):
+                    self.pack('I', -42)
+
+            struct.x = C()
+            """
+        rc, stdout, stderr = assert_python_ok("-c", code)
+        self.assertEqual(rc, 0)
+        self.assertEqual(stdout.rstrip(), b"")
+        self.assertIn(b"Exception ignored in:", stderr)
+        self.assertIn(b"C.__del__", stderr)
 
 class UnpackIteratorTest(unittest.TestCase):
     """