Bugfix #218 - #230

Bug fix:
Changes bootstraps_color_by_group  check in misc_tools to work the same way for both paired data and unpaired data (since unpaired data was working), with the exception of delta-delta and Sankey

- Support `custom_palette` dicts together with `color_col`, which still raised
  "The `custom_palette` dictionary is not supported when `color_col` is not
  None." The palette is now keyed by the `color_col` categories, as the
  `plot()` docstring already describes, with an explicit error listing any
  category left without a colour.

- Collapse the paired branch of `get_color_palette` to the single condition it
  encodes. The nested branches recomputed `color_groups` and re-raised a
  KeyError that the block directly above already handles; rendered output is
  unchanged.

- Add `nbs/tests/test_color_palette.py`. Paired + `color_col` +
  `custom_palette` had no coverage at all, which is how #207 broke it: the two
  existing tests that pass both arguments use an unpaired object, so they never
  reach this branch.

- Read README.md as UTF-8 in setup.py rather than replacing its curly quotes.
  The install failure came from the locale-dependent `open()`, so any future
  non-ASCII character would have brought it back.