Update allow_lazy usage example for Python 3

[Alexey Boriskin]

There is a line in the usage example of allow_lazy:
fancy_utility_function = allow_lazy(fancy_utility_function, unicode)
It would be better to use six.text_type instead of unicode, because unicode does not exist of python3

[Baptiste Mispelon]

There's a lot of places in the documentation with code examples that don't work in python3 (see #19841 for example).

I think we need to decide what to do about it in general.

From what I understand, the current position is that python3 support is "experimental", so the documentation stays python2-oriented. This will surely change at some point, and probably soon.

[Claude Paroz]

Python 3 support will not be called experimental in 1.6, so yes, a fix is welcome here. However, I'm not sure if always using six is a good idea. It's the right thing to do to support both Python 2 and Python 3, but most users will probably target either one or the other. An alternative could be adding a str/unicode note somewhere in the docs and refer to that note each time we are using unicode in examples.

[Baptiste Mispelon]

For what it's worth, I'm +1 on claudep's proposal.

[Claude Paroz]

​https://github.com/django/django/pull/1330

[Tim Graham]

Left minor comments on the PR, should be good to go without another review though, thanks!

[Claude Paroz <claude@…>]

In 7442eb1a242ecf9d186d4e7de1b94e360e04782d:

Fixed #20224 -- Update docs examples which mention unicode

Thanks Marc Tamlyn and Tim Graham for the review.

[Claude Paroz <claude@…>]

In a9dd6221af2148410c8a26dcbafd1ff8cc0fb107:

[1.6.x] Fixed #20224 -- Update docs examples which mention unicode

Thanks Marc Tamlyn and Tim Graham for the review.
Backport of 7442eb1a24 from master.