`$PWD` isn't portable (POSIX requires it, but traditional shells don't support it).

Using export and setting a value at the same time also isn't portable (again, POSIX requires it but older shells don't support it).

It's more robust to use `&&` after the `cd`, so the rule fails if the `docs` directory doesn't exist rather than generating documentation in the wrong directory.

I'd also avoid the trailing slash on `docs/` - some tools can be a bit fussy about them (especially on the commercial Unix platforms) - I don't recall particularly if `cd` dislikes them anywhere, but it seems cleaner without the slash anyway.

Putting that all together, my suggestion would be:

```
cd docs && PYTHONPATH=..:$$PYTHONPATH sphinx-build -b html -d _build/doctrees .  _build/html
```

(A relative path in PYTHONPATH works, as we already use `.` in the `run-python-test` script.)