Update pprint version to 0.7.1?

[Quafadas]

I attempted some version updates on the Almond codebase and submitted a PR
almond-sh/almond#902

Which failed in CI. @sideeffffect has been kind enough to look at the underlying reason, which would appear to be that ammonite / mdoc now rely on incompatible versions of PPrint. Ammonite = 0.7.1, mdoc = 0.6.0

The conclusion was to file an issue here to note the "update opportunity". My attempts to change something on a local clone resulted in confusion - I do not think it worth submitting those here.