gh-123339: Fix cases of inconsistency of __module__ and __firstlineno__ in classes

* 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 the latters. 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.

Author: serhiy-storchaka

Misc/NEWS.d/next/Core_and_Builtins/2024-09-02-20-36-45.gh-issue-123339.QcmpSs.rst

@@ -0,0 +1,3 @@
+Setting the :attr:`!__module__` attribute for a class now removes the
+``__firstlineno__`` item from the type's dict, so they will no longer be
+inconsistent.

Misc/NEWS.d/next/Library/2024-09-02-20-34-04.gh-issue-123339.czgcSu.rst

@@ -0,0 +1,4 @@
+Fix :func:`inspect.getsource` for classes in :mod:`collections.abc` and
+:mod:`decimal` (for pure Python implementation) modules.
+:func:`inspect.getcomments` now raises OSError instead of IndexError if the
+``__firstlineno__`` value for a class is out of bound.