build: add autoconf python3 to Ubuntu dependencies

Closes: https://github.com/giuseppe/crun/issues/55

Signed-off-by: Giuseppe Scrivano <giuseppe@scrivano.org>

README.md

@@ -55,7 +55,7 @@ On Ubuntu:
 ```shell
 apt-get install -y make git gcc build-essential pkgconf libtool \
    libsystemd-dev libcap-dev libseccomp-dev libyajl-dev libselinux1-dev \
-   go-md2man libtool
+   go-md2man libtool autoconf python3
 ```
 
 Unless you are also building the Python bindings, Python is needed