add Python 3 source

source/python3/README

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+Python is an interpreted, interactive, object-oriented programming
+language that combines remarkable power with very clear syntax.
+Python's basic power can be extended with your own modules written in C
+or C++.  Python is also adaptable as an extension language for existing
+applications.
+
+Python 3 (a.k.a. "Python 3000" or "Py3k") is a new version of the
+language that is incompatible with the 2.x line of releases.  The
+language is mostly the same, but many details, especially how built-in
+objects like dictionaries and strings work, have changed considerably,
+and a lot of deprecated features have finally been removed.  Also, the
+standard library has been reorganized in a few prominent places.
+
+It is safe to install this on a system which already has 2.x version of
+Python.
+
+If you'd like to have HTML docs installed, get them from
+<https://docs.python.org/3/download.html> (HTML format, .tar.bz2
+archive).