checker of using python2

[MURAOKA Taro]

Hi the list and Bram.

I suggest to add checker of using Python 2.x

Python 2 will be retire 2020, and it is not so far.
https://pythonclock.org/
To stop using Python 2.x suddenly is not easy for Vim users.
Because it is difficult to know which scripts are using Python 2.
So vim may help to detect it.

I have wrote a patch for proof of the concept.
Please check it:
https://gist.github.com/koron/90f6b56b73c211e059a3b8ab1db335a9

My patch do three things:

1. Show a waring message at only once when Python 2.x is used.
2. The warning could be suppressed by 'nopy2warn' option.
3. :py2usingfrom shows list of files which use Python 2.x

Names for command and option which I added are not good, I think.
Please replace those by better names.

Best.
--
MURAOKA Taro <koron....@gmail.com>

[Bram Moolenaar]

Can be useful, if the user cares about avoiding Python 2.
But it should not warn by default.

It should be possible to add a flag in scriptitem_T. And the line
number would be useful too. The sourcing_lnum has the value.

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[MURAOKA Taro]

Thank you for comments.
I'll try to improve ;)

--
MURAOKA Taro <koron....@gmail.com>