Update unicode documentation for Python 3

[Aymeric Augustin]

The "Unicode data" page doesn't make sense for people who started on Python 3 and who have always had the luxury of a str type that works.

Also it assumes that Python's default charset is ASCII, while it is UTF-8 now.

This page should be renamed to "Encoded data" and, instead of explaining how unicode strings are different from (Python 2) regular strings, it should explained how encoded strings are different from (Python 3) regular strings and how external systems deal with them.

There's also a handful of references to "Unicode strings" in other documentation pages that should be changed to just "strings".

[Claude Paroz]

This ​PR is dealing with the unicode term replacement with "string". But as I commented on the PR, the "Unicode data" page does indeed need to be rewritten.

[Tim Graham <timograham@…>]

In d1bab24:

Refs #23919, #27778 -- Removed obsolete mentions of unicode.

[Tim Graham]

A ​PR starts this but hasn't done much rewriting yet.

[Tim Graham <timograham@…>]

In f5d9ee1:

Refs #27778 -- Removed "The database API" section from "Unicode data" docs.

Support for passing bytestrings to the database API was removed in
301de774c21d055e9e5a7073e5bffdb52bc71079.

[Tim Graham <timograham@…>]

In d1b25a64:

[2.1.x] Refs #27778 -- Removed "The database API" section from "Unicode data" docs.

Support for passing bytestrings to the database API was removed in
301de774c21d055e9e5a7073e5bffdb52bc71079.
Backport of f5d9ee11a2c76ba1cf7e3424d15286da121a004b from master

[GitHub <noreply@…>]

In 505b7b6:

Refs #27778 -- Removed reference to ASCII usernames in django.contrib.auth.models.User docs.

[Mariusz Felisiak <felisiak.mariusz@…>]

In fc0e087:

[3.0.x] Refs #27778 -- Removed reference to ASCII usernames in django.contrib.auth.models.User docs.

Backport of 505b7b616320b8d5bbc83d0dbbb3aec3a58ba0c9 from master

[Basant Babu Bhandari]

working on it

[Basant Babu Bhandari]

I want to work on it can some body help me out