Replace "sudo pip install" recommendation with virtualenv

[Tim Graham]

We've removed most of "sudo pip" from the docs, but two instances remain:

docs/topics/install.txt:   command ``sudo pip install Django`` at the shell prompt. If you're using
docs/topics/install.txt:        $ sudo pip install -e django-trunk/

We should probably replace this recommendation with a requirement to use virtualenv.

​django-developers thread which spawned this ticket.

[Yask Srivastava]

It's already mentioned in the doc as a second pointer:

"2. (optional) Take a look at virtualenv_ and virtualenvwrapper_. These tools provide isolated Python environments, which are more practical than installing packages systemwide. They also allow installing packages without administrator privileges. It's up to you to decide if you want to learn and use them."

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In a523d94:

Fixed #25376 -- Required virtualenv in installation instructions.

Thanks Anjul Tyagi for some of the draft text.

[Tim Graham <timograham@…>]

In e21c6f34:

[1.9.x] Fixed #25376 -- Required virtualenv in installation instructions.

Thanks Anjul Tyagi for some of the draft text.

Backport of a523d94b4581ec41c86f5bbaaffdf5bb6b819c2e from master

[Tim Graham <timograham@…>]

In f3ce888:

[1.8.x] Fixed #25376 -- Required virtualenv in installation instructions.

Thanks Anjul Tyagi for some of the draft text.

Backport of a523d94b4581ec41c86f5bbaaffdf5bb6b819c2e from master