…__firstlineno__ in classes (pythonGH-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.
(cherry picked from commit 69a4063)

Co-authored-by: Serhiy Storchaka <[email protected]>