Introduce code linters to the repo

[leofang]

As #153 is being reviewed, I notice we are struggling in sticking to some formatting preferences like

• the number of empty lines between code blocks
• the line endings (which shows up in particular at the end of the file diff)
• the import order

My rule of thumb: Once the team size is >=3 we should add a code linter, ex: Ruff, plus optionally a pre-commit hook, so that files are linted and updated before checked in. We can further add a pre-commit CI to enforce the checks & code cleanness. Both sides of code review participants should focus only on the functionality/correctness, not formatting.

[leofang]

Let's focus on pure Python .py files for now; the majority of Cython .pxd/.pyx files are auto-gen'd and not meant to be "human friendly" anyway.