Change the extension of the README, so it will get rendered by GitHub

Also, removes README.python3, which doesn't provide any extra information
that's not already found in the README.

README.Python3

@@ -1,33 +0,0 @@
-Python3
-=======
-
-Compatibility
--------------
-
-Please, consider python3 >= 3.2 only.
-
-
-Approach
---------
-
-We maintain a code base that is Python 2/3 compatible simultaneously.
-
-In order to run pylint locally, you have to install the dependencies::
-
-  easy_install-3.2 logilab-common
-  easy_install-3.2 astroid
-  easy_install-3.2 six
-
-
-Debian
-------
-
-For the Debian packaging, you can use the debian.py3k/ content against
-the debian/ folder::
-
-  cp debian.py3k/* debian/
-
-
-Resources
----------
-https://docs.python.org/3/howto/pyporting.html