Switch to using the fork & fix of `pdb++`, `pdbp`:
https://github.com/mdmintz/pdbp

Allows us to sidestep a variety of issues that aren't being maintained
in the upstream project thanks to the hard work of @mdmintz!

We also include some default settings adjustments as per recent
development on the fork:

- sticky mode is still turned on by default but now activates when
  a using the `ll` repl command.
- turn off line truncation by default to avoid inter-line gaps when
  resizing the terimnal during use.
- when using the backtrace cmd either by `w` or `bt`, the config
  automatically switches to non-sticky mode.
