Prefer readthedocs.io instead of readthedocs.org for doc links

Read the Docs moved hosting to readthedocs.io instead of readthedocs.org. Fix
all links in the project.

For additional details, see:

https://blog.readthedocs.com/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.

Update badge link per the documentation:

https://read-the-docs.readthedocs.io/en/latest/badges.html

> If you leave the version argument off, it will default to your latest
> version. This is probably best to include in your README, since it
> will stay up to date with your Read the Docs project:

Author: jdufresne

README.rst

@@ -24,7 +24,7 @@ Accessing protected resources using requests_oauthlib is as simple as:
 Before accessing resources you will need to obtain a few credentials from your
 provider (e.g. Twitter) and authorization from the user for whom you wish to
 retrieve resources for. You can read all about this in the full
-`OAuth 1 workflow guide on RTD <http://requests-oauthlib.readthedocs.org/en/latest/oauth1_workflow.html>`_.
+`OAuth 1 workflow guide on RTD <https://requests-oauthlib.readthedocs.io/en/latest/oauth1_workflow.html>`_.
 
 The OAuth 2 workflow
 --------------------
@@ -37,7 +37,7 @@ Fetching a protected resource after obtaining an access token can be extremely
 simple. However, before accessing resources you will need to obtain a few
 credentials from your provider (e.g. Google) and authorization from the user
 for whom you wish to retrieve resources for. You can read all about this in the
-full `OAuth 2 workflow guide on RTD <http://requests-oauthlib.readthedocs.org/en/latest/oauth2_workflow.html>`_.
+full `OAuth 2 workflow guide on RTD <https://requests-oauthlib.readthedocs.io/en/latest/oauth2_workflow.html>`_.
 
 Installation
 -------------
@@ -52,7 +52,7 @@ To install requests and requests_oauthlib you can use pip:
    :target: https://travis-ci.org/requests/requests-oauthlib
 .. |coverage-status| image:: https://img.shields.io/coveralls/requests/requests-oauthlib.svg
    :target: https://coveralls.io/r/requests/requests-oauthlib
-.. |docs| image:: https://readthedocs.org/projects/requests-oauthlib/badge/?version=latest
+.. |docs| image:: https://readthedocs.org/projects/requests-oauthlib/badge/
    :alt: Documentation Status
    :scale: 100%
-   :target: https://readthedocs.org/projects/requests-oauthlib/
+   :target: https://requests-oauthlib.readthedocs.io/

docs/conf.py

@@ -251,7 +251,7 @@
 
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
-    'python': ('http://python.readthedocs.org/en/latest/', None),
-    'requests': ('http://requests.readthedocs.org/en/latest/', None),
-    'oauthlib': ('http://oauthlib.readthedocs.org/en/latest/', None),
+    'python': ('https://python.readthedocs.io/en/latest/', None),
+    'requests': ('https://requests.readthedocs.io/en/latest/', None),
+    'oauthlib': ('https://oauthlib.readthedocs.io/en/latest/', None),
 }