Add requirements.txt

- Allows simplifying local setup instructions without docker.
- Also use python venv (which is builtin with standard python3).
- Install as much stuff to python venv as possible to make things more
  cross-plaform.
- Fix some minor bash snippets which were not labled (and thus were
  being treated as cpp snippets)

Signed-off-by: Ayush Singh <ayushdevel1325@gmail.com>