Support private PyPI repositories

This issue is related to the following conversation from fast.ai forum: https://forums.fast.ai/t/nbdev-for-private-pypi-repositories-and-poetry-package-management/100523

# Goal of the issue

1. To gauge the interest in nbdev supporting private PyPi repos (e.g. GCP Artifact Registry).
2. To discuss the best way to integrate private PyPi repo support into nbdev.

If there's interest, I'd be happy to make the contribution.

# Problem

nbdev does not support dependencies from private PyPi repositories.

# Workaround

Changing the following line in [nbdev CI workflow](https://github.com/fastai/workflows/blob/master/nbdev-ci/action.yml):

```bash
test -f setup.py && pip install -e ".[dev]"
```

to:

```bash
test -f setup.py && pip install --extra-index-url https://$PYPIUSER:$PYPIPASSWORD@$PYPISERVER -e ".[dev]"
```

fixes the issue.