This allows findsource() to work in doctests.

A patch from Dirkjan Ochtman.

Original fix by 'rep', final patch (with tests) by Xuanji Li.

Rather than wrapping the C _isdir function in a Python function,
just import the C _isdir function directly. Additionally, add in the
docstring which was left out.

Extract of the commit message:

  Fix usage of :option: in the docs (#9312).

  :option: is used to create a link to an option of python, not to mark
  up any instance of any arbitrary command-line option.  These were
  changed to ````.

Issue #12080: Fix a performance issue in Decimal._power_exact that caused some corner-case Decimal.__pow__ calls to take an unreasonably long time.

test_multibytecodec_support. Improve also error message of the
test_errorhandle() of TestBase.

* Use concrete example for dir() and eliminate the distracting doctest directives.
* Add a pure python equivalent for enumerate()
* Modify the enumerate() example to demonstrate the start argument
* Remove incorrect reference the *iterable* in the enumerate() docs.
* Downgrade the comments on input() from a warning to a note.
* Fix the iter() example to use the empty string as the terminating
  condition for file.readline().  Also, the old example was broken
  because readline() results include a newline, so 'STOP\n' would have
  been the correct terminating condition.  Even with that fix, the
  STOP example was fragile and would have lead to infinite loops with
  malformed inputs.
* Do not refer to classmethod as being "more advanced" than staticmethod.