Issue #15767: Introduce ModuleNotFoundError, a subclass of

ImportError.

The exception is raised by import when a module could not be found.
Technically this is defined as no viable loader could be found for the
specified module. This includes ``from ... import`` statements so that
the module usage is consistent for all situations where import
couldn't find what was requested.

This should allow for the common idiom of::

  try:
    import something
  except ImportError:
    pass

to be updated to using ModuleNotFoundError and not accidentally mask
ImportError messages that should propagate (e.g. issues with a
loader).

This work was driven by the fact that the ``from ... import``
statement needed to be able to tell the difference between an
ImportError that simply couldn't find a module (and thus silence the
exception so that ceval can raise it) and an ImportError that
represented an actual problem.

Lib/pydoc.py

@@ -317,7 +317,7 @@ def safeimport(path, forceload=0, cache={}):
         elif exc is SyntaxError:
             # A SyntaxError occurred before we could execute the module.
             raise ErrorDuringImport(value.filename, info)
-        elif exc is ImportError and value.name == path:
+        elif issubclass(exc, ImportError) and value.name == path:
             # No such module in the path.
             return None
         else: