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.

README.adoc

@@ -36,7 +36,7 @@ You need to have a GNU/Linux distribution to be able to compile Turris OS and pa
 To install all dependencies for Debian distribution you can use:
 
 --
-  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python file rsync gcc-multilib g++-multilib
+  apt install ca-certificates git build-essential zlib1g-dev gawk libssl-dev subversion unzip libncurses-dev wget python python3 file rsync gcc-multilib g++-multilib
 --
 
 For other distributions, we suggest to look at https://openwrt.org/docs/guide-developer/build-system/install-buildsystem[OpenWrt documentation].