- replace cython0 with cython in makedepends (there is no compatibility problem
  with the latest version)
- add python to optdepends
- add bash to depends (previously it was an implicitly satisfied dependency)
- remove ethtool from depends (not needed since 2020, iproute2 has `rdma link add`)

rdma-core release tarballs are not signed and the releases are made by
different people these days.

- update license to SPDX, list all licenses per COPYING.md
- reformat dependencies for easier handling
- add glibc to depends
- modernize cmake commands
- remove python tests installed to /usr/share/doc/
- try to install all license files on a best-effort basis