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.

Lib/inspect.py

@@ -970,10 +970,12 @@ def findsource(object):
 
     if isclass(object):
         try:
-            firstlineno = vars(object)['__firstlineno__']
+            lnum = vars(object)['__firstlineno__'] - 1
         except (TypeError, KeyError):
             raise OSError('source code not available')
-        return lines, firstlineno - 1
+        if lnum >= len(lines):
+            raise OSError('lineno is out of bounds')
+        return lines, lnum
 
     if ismethod(object):
         object = object.__func__