pythongh-106078: Convert _decimal types to heap types (python#106079)

- Establish global state struct
- Convert static types to heap types and add them to global state:
    * PyDecContextManager_Type
    * PyDecContext_Type
    * PyDecSignalDictMixin_Type
    * PyDec_Type
- Add to global state:
    * PyDecSignalDict_Type
    * DecimalTuple

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
Co-authored-by: Erlend E. Aasland <erlend@python.org>

Lib/test/test_decimal.py

@@ -34,7 +34,8 @@
 import locale
 from test.support import (is_resource_enabled,
                           requires_IEEE_754, requires_docstrings,
-                          requires_legacy_unicode_capi, check_sanitizer)
+                          requires_legacy_unicode_capi, check_sanitizer,
+                          check_disallow_instantiation)
 from test.support import (TestFailed,
                           run_with_locale, cpython_only,
                           darwin_malloc_err_warning, is_emscripten)
@@ -5681,6 +5682,24 @@ def test_maxcontext_exact_arith(self):
             self.assertEqual(Decimal(4) / 2, 2)
             self.assertEqual(Decimal(400) ** -1, Decimal('0.0025'))
 
+    def test_c_immutable_types(self):
+        SignalDict = type(C.Context().flags)
+        SignalDictMixin = SignalDict.__bases__[0]
+        ContextManager = type(C.localcontext())
+        types = (
+            SignalDictMixin,
+            ContextManager,
+            C.Decimal,
+            C.Context,
+        )
+        for tp in types:
+            with self.subTest(tp=tp):
+                with self.assertRaisesRegex(TypeError, "immutable"):
+                    tp.foo = 1
+
+    def test_c_disallow_instantiation(self):
+        ContextManager = type(C.localcontext())
+        check_disallow_instantiation(self, ContextManager)
 
 @requires_docstrings
 @requires_cdecimal