bpo-13349: Improve the error message for tuple.index

[Zaharid]

When the required value is not found, use the same template as
list.index and display the missing value instead of 'x'. The original issue with the discussion is: https://bugs.python.org/issue13349

[the-knights-who-say-ni]

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[mention-bot]

@Zaharid, thanks for your PR! By analyzing the history of the files in this pull request, we identified @serhiy-storchaka, @rhettinger and @tiran to be potential reviewers.

[louisom]

@Zaharid You will need to put the bpo issue number at the front of the title: bpo-13349: Improve..., also, you will need to signed the CLA first.

[DimitrisJim]

Hi, @Zaharid and thanks for the patch. I had already created a PR for this issue here #876 already, though :-).

[berkerpeksag]

Closing this as a duplicate of #876.