Use python3 in CI and documented build requirements

This has become possible with OpenWrt 21.02 and while the Dockerfile
already received that updated, we forgot to update the CI and the
documentation.

contrib/actions/install-dependencies.sh

@@ -3,6 +3,6 @@
 set -e
 
 apt-get -y update
-apt-get -y install git subversion build-essential python gawk unzip libncurses5-dev zlib1g-dev libssl-dev wget time qemu-utils
+apt-get -y install git subversion build-essential python3 gawk unzip libncurses5-dev zlib1g-dev libssl-dev wget time qemu-utils
 apt-get -y clean
 rm -rf /var/lib/apt/lists/*

docs/user/getting_started.rst

@@ -29,7 +29,7 @@ freshly installed Debian Stretch system the following packages are required:
 
 * `git` (to get Gluon and other dependencies)
 * `subversion`
-* `python` (Python 3 doesn't work)
+* `python3`
 * `build-essential`
 * `gawk`
 * `unzip`