Convert readthedocs links for their .org -> .io migration for hosted projects

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

docs/dev/virtualenvs.rst

@@ -122,7 +122,7 @@ control by adding it to the ignore list.
 virtualenvwrapper
 -----------------
 
-`virtualenvwrapper <http://virtualenvwrapper.readthedocs.org/en/latest/index.html>`_
+`virtualenvwrapper <https://virtualenvwrapper.readthedocs.io/en/latest/index.html>`_
 provides a set of commands which makes working with virtual environments much
 more pleasant. It also places all your virtual environments in one place.
 
@@ -134,7 +134,7 @@ To install (make sure **virtualenv** is already installed):
   $ export WORKON_HOME=~/Envs
   $ source /usr/local/bin/virtualenvwrapper.sh
 
-(`Full virtualenvwrapper install instructions <http://virtualenvwrapper.readthedocs.org/en/latest/install.html>`_.)
+(`Full virtualenvwrapper install instructions <https://virtualenvwrapper.readthedocs.io/en/latest/install.html>`_.)
 
 For Windows, you can use the `virtualenvwrapper-win <https://github.com/davidmarble/virtualenvwrapper-win/>`_.
 
@@ -206,7 +206,7 @@ Other useful commands
 ``lssitepackages``
   Shows contents of :file:`site-packages` directory.
 
-`Full list of virtualenvwrapper commands <http://virtualenvwrapper.readthedocs.org/en/latest/command_ref.html>`_.
+`Full list of virtualenvwrapper commands <https://virtualenvwrapper.readthedocs.io/en/latest/command_ref.html>`_.
 
 virtualenv-burrito
 ------------------