docs: clarify Python 3 version guidance

[swaroopch]

The README tells beginners that Python 2 is "older and more common" and promises guidance for adapting to it. This change clearly presents the book as Python 3 material and directs readers to use Python 3 for the examples and new programs.

Closely related guidance in about_python.md, more.md, and the Unicode note in io.md now treats Python 2 as legacy context and frames migration toward Python 3. Existing historical resource links, testimonials, revision history, examples, headings, and anchors are preserved.

Validation: repository-wide search for Python 2 and migration guidance; git diff --check; verified that the README's testimonial section and everything following it are byte-for-byte unchanged, and that headings, anchors, and resource lists are preserved in all changed files. No executable code changed; the book build was not run.

Recreation prompt

If you prefer to implement the intent independently rather than use this code diff:

Update swaroopch/byte-of-python so its README clearly identifies the book as Python 3 material and tells beginners to use Python 3, removing the claim that Python 2 is more common and the promise to adapt examples to Python 2. Inspect closely related Python 2 migration and compatibility guidance in the repository, and correct clearly stale instructional framing so Python 2 is treated as historical or legacy context and migration is toward Python 3. Keep the change focused on prose: preserve historical testimonials, revision history, historical resource links, executable examples, headings, and anchors. Verify the diff has no whitespace errors and that preserved content remains unchanged.