doc: Reinstate requirements.txt for RTD
doc: Reinstate requirements.txt for RTD

gareth8118/spreads,miloh/spreads,DIYBookScanner/spreads,miloh/spreads,gareth8118/spreads,DIYBookScanner/spreads,nafraf/spreads,nafraf/spreads,DIYBookScanner/spreads,nafraf/spreads,gareth8118/spreads,miloh/spreads,adongy/spreads,adongy/spreads,adongy/spreads