[pyos][docs] `CONTRIBUTING.md` mentions `requirements.txt` but no such file exists

[sneakers-the-rat]

Part of: pyOpenSci/software-submission#267 (comment)

Hallucinated instructions are bad for external contributors:

OneCite/CONTRIBUTING.md

Lines 46 to 48 in 12b1dea

	# Or install from requirements files
	pip install -r requirements.txt
	pip install -r requirements-dev.txt