Getting a USB PD 3.0 port to output a constant voltage
To get 20V out of a USB C power supply you need a USB power delivery controller designed to work with USB C (there was a version of USB power delivery for A/B connectors but afaict it never took off). 

From some searching it seems the STUSB4500 should fill your requirements. It supports 20V mode and once the non-volatile memory is programmed it can work without support from a host processor.