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.

1 files changed, 4 insertions, 1 deletions

diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 5ce6bf1..513199d 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -1080,7 +1080,10 @@ Special attributes
        .. versionadded:: 3.13
 
    * - .. attribute:: type.__firstlineno__
-     - The line number of the first line of the class definition, including decorators.
+     - The line number of the first line of the class definition,
+       including decorators.
+       Setting the :attr:`__module__` attribute removes the
+       :attr:`!__firstlineno__` item from the type's dictionary.
 
        .. versionadded:: 3.13