deprecation warning. Closes #414

There is no reason for pip to automatically install dependencies
of requested packages, because all of the dependencies have already
been resolved and explicitly listed in the input requirements.txt
file generated by `poetry export --dev`.

In addition to making the deployment a little bit more efficient,
this also works around a bug in pip:
[pip#9644](https://github.com/pypa/pip/issues/9644).

- Fix failing detchar test for python3.9
- Restrict ligo.em-bright < 1.0 until SCCB approval. See !854
Using importlib resources on a data path leads to IsADirectory
error. Replaced with pathlib.

Reason for manual update of hash: https://github.com/python-poetry/poetry/issues/4523

- bump p-astro to a pre-release v1.0.0dev1, stripping all em-bright components
- add ligo.em-bright dependency