files cannot be written.

ImportError if __name__ is not defined on a package.

Thanks to Armin Rigo for the bug report and diagnosing the cause.

Patch by Christie Wilson.

Patch by Isaac Schwabacher.

The concept of .pyo files no longer exists. Now .pyc files have an
optional `opt-` tag which specifies if any extra optimizations beyond
the peepholer were applied.

Ensure that the module is unloaded to be able to run the test more than once,
and to not leak memory.

filenames anymore on Windows.

...`` when an ImportError occurs.

Other cases had already been switched over to using the repr.

Thanks to Tomasz Maćkowiak for the patch.

importlib._bootstrap._get_sourcefile().

Thanks to its only use by the C API, it was never properly tested
until now.

Thanks to Neal Norwitz for discovering the bug and Madison May for the patch.

ModuleNotFoundError.

To make sure there is no issue with code that is both Python 2 and 3
compatible, there are no plans to remove the module any sooner than
Python 4 (unless the community moves to Python 3 solidly before then).

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.

test_imp.

raise an ImportError (rather than fatal) when __import__ is not found in __builtins__ (closes #17867)