gh-123339: Fix cases of inconsistency of __module__ and __firstlineno__ in classes (GH-123613)

* Setting the __module__ attribute for a class now removes the
  __firstlineno__ item from the type's dict.
* The _collections_abc and _pydecimal modules now completely replace the
  collections.abc and decimal modules after importing them. This
  allows to get the source of classes and functions defined in these
  modules.
* inspect.findsource() now checks whether the first line number for a
  class is out of bound.

Objects/typeobject.c

@@ -1435,6 +1435,9 @@ type_set_module(PyTypeObject *type, PyObject *value, void *context)
     PyType_Modified(type);
 
     PyObject *dict = lookup_tp_dict(type);
+    if (PyDict_Pop(dict, &_Py_ID(__firstlineno__), NULL) < 0) {
+        return -1;
+    }
     return PyDict_SetItem(dict, &_Py_ID(__module__), value);
 }