After discussing this with @leouieda, we decided to set local_scheme = "no-local-version" by default. This makes easier to upload to Test PyPI without any shananigans on the Actions workflow and we actually don't need the version to have the commit hash or a date, just to know that we are on an untagged commit. More information can be obtained with git status.

After this decision, setuptools_scm will raise error if we try to install the dev version of Harmonica though pip in the following way:
pip install https://github.com/fatiando/harmonica.

Instead, we would need to do so with:

pip install git+https://github.com/fatiando/harmonica

But this requires git to be installed (it clones the repository instead of downloading a tarball).

So we need to update installing instructions for latest development version to use pip install git+https://github.com/fatiando/harmonica instead of pip install https://..., and add a note that this step now requires git installed.

After discussing this with @leouieda, we decided to set local_scheme = "no-local-version" by default. This makes easier to upload to Test PyPI without any shananigans on the Actions workflow and we actually don't need the version to have the commit hash or a date, just to know that we are on an untagged commit. More information can be obtained with git status.

After this decision, setuptools_scm will raise error if we try to install the dev version of Harmonica though pip in the following way:
pip install https://github.com/fatiando/harmonica.

Instead, we would need to do so with:

pip install git+https://github.com/fatiando/harmonica

But this requires git to be installed (it clones the repository instead of downloading a tarball).

So we need to update installing instructions for latest development version to use pip install git+https://github.com/fatiando/harmonica instead of pip install https://..., and add a note that this step now requires git installed.

So it seems that not adding the local version string gets in conflict with Pooch, specifically on detecting if Pooch should use version or version_dev for creating the cache folder:

https://github.com/fatiando/pooch/blob/1f60c6ffe143a233bf15b5d1d1ac5acc648504ba/pooch/core.py#L380-L381

https://github.com/fatiando/pooch/blob/1f60c6ffe143a233bf15b5d1d1ac5acc648504ba/pooch/utils.py#L108-L151

I'll try to restore the default behaviour of local_scheme and add some shananingans with sed on the Actions workflow for publishing to TestPyPI.