Remove deprecated pyjet dependency in favor of fastjet

[matthewfeickert]

SHAPER currently requires pyjet as a dependency

SHAPER/README.md

Line 26 in 0a18be2

* [pyjet](https://github.com/scikit-hep/pyjet): A package for jet clustering, needed for default observable definitions.

However, pyjet is unsupported and deprecated (c.f. scikit-hep/pyjet#84) and we are asking that projects stop using it in favor of https://github.com/scikit-hep/fastjet instead.