Get rid of the python2 build dependency.

Python 2 has been EOL since 2019, and has recently been removed from the Arch repositories, so continuing to support it has become a major hassle for me, especially on the OBS. The quick and dirty fix is to just build the DISIS cwiid library without it, which seems to be the only part of the source which at present still depends on Python 2 at build time. This lets us keep the DISIS externals as they are, while also getting rid of the python2 build dependency.

TBH, I'm not sure whether the disis_wiimote external requires the cwiid Python interface in any way. FWIW, I can't find any Python references in the DISIS external source, so I think that it should be fine without it. But I couldn't actually test this since the disis_wiimote object stopped working for me ever since bluez 5 (i.e., a long time ago).

I've run test builds of this branch on the OBS, all fine on Arch, Debian, Fedora, Raspbian, and Ubuntu. The proposed changeset should only affect Linux systems, but I also made sure that the source still compiles and runs on Mac and Windows anyway.