Add requirments.txt to list development requirements

This MR reinstates a requirements.txt file which lists non-standard development requirements that are needed to create software environments in which to develop, but which shouldn't considered 'official' requirements of the project.

The distinction is that this project needs hermes.aeriel, which is what should go into pyproject.toml (technically with a . separator, not a -). In practice, that's not installable from PyPI (but might be in the future), so the requirements.txt provides a means for the user to manually build it into their software environment. If that makes any sense.

After this patch you still can't just pip install . from a clone of this project, but you can run pip install -r requirements.txt . to combine the requirements.

This also means that @vasileios.skliris could upload an mpl_pipeline wheel to PyPI, which users could use as follows:

pip install -r /path/to/mpl_pipeline/requirements.txt
pip install mly_pipeline

(i.e. by manually pre-installing hermes.<thing> using the requirements.txt file. If that also makes any sense.