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/decimal.py

@@ -103,6 +103,7 @@ try:
     from _decimal import __version__  # noqa: F401
     from _decimal import __libmpdec_version__  # noqa: F401
 except ImportError:
-    from _pydecimal import *
-    from _pydecimal import __version__  # noqa: F401
-    from _pydecimal import __libmpdec_version__  # noqa: F401
+    import _pydecimal
+    import sys
+    _pydecimal.__doc__ = __doc__
+    sys.modules[__name__] = _pydecimal