codespell: install via pip instead of apt

[aabadie]

Unfortunately the version of codespell installed by apt doesn't contain some of the options used in RIOT-OS/RIOT#12232

This PR is installing codespell via pip3 with the latest current version (1.16) to be sure all required features can be used.

[aabadie]

The version installed is the right one:

$ docker build . -t riot/riotbuild
$ docker run -ti --rm riot/riotbuild /bin/bash
root@3ea93c11e4c3:~# codespell --version
1.16.0

[kaspar030]

ACK.