README: add Python3 to requirements
This is required for OpenWrt master (which is going to become OpenWrt 20.xx/21.xx) once it is released. Otherwise, you will get this error: Build dependency: Please install Python >= 3.5
Python2 needs to be there for OpenWrt 19.07.
According to https://openwrt.org/docs/guide-developer/build-system/install-buildsystem Same as https://gitlab.nic.cz/turris/turris-build/-/merge_requests/315#note_188602