nf and qqbar into python-flint?

[dimpase]

We (in sagemath) have an interest in bringing Flint's number fields, i.e. nf and qqbar into sagemath; I prefer this happening via python-flint.

We also have a current GSoC student @Madhu18S looking into this sagemath/sage#42261

@fredrik-johansson

Originally posted by @dimpase in #53