Fixes on the building of python bindings documentaion

following olly comments:
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$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.)
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