Add Pythonlings interactive Python exercises

[abhiksark]

Summary

Add Pythonlings to Programming Languages > Python. Pythonlings is a freely available, terminal-based learning resource where beginners learn Python by fixing small broken programs.

Contribution guideline checks

• The resource is text-and-code based, not a video course.
• The resource is freely available from its author under the MIT License.
• The resource is an independent project, not an official Python site.
• The entry is in the Python section and in lexicographic order.
• The change adds one resource using the required link format, names the author, and omits a trailing slash.
• Searches of the README and open and closed issues and pull requests found no existing entry.

Validation

• git diff --check
• Reviewed the final README.md diff and placement.
• Confirmed the project URL returns HTTP 200.

Maintainer disclosure

I am the author and maintainer of Pythonlings, and I am submitting a project I maintain.