Add py and py3 to Python aliases

[rvben]

Summary

Add py as an alias for Python, matching how other popular languages handle short aliases.

Motivation

Python is the only major language without its common short alias:

Language	Short Alias
JavaScript	js ✅
TypeScript	ts ✅
Ruby	rb ✅
Rust	rs ✅
Python	py ❌

In #4026 (2018), adding py was discussed and the response was:

"Let's keep the python3 alias only for now. We can always add new aliases if/when we see them being used."

Seven years later, py is extensively used in code blocks across GitHub. The alias is so expected that GitHub's markdown renderer added extension-based fallback to support ```py even without it being an official alias.

Impact

• Tools using Linguist's find_by_alias() will recognize py
• Aligns official aliases with GitHub's rendering behavior
• Zero risk: no other language uses py

Changes

Add py to the Python aliases in lib/linguist/languages.yml.