Generate version string on _version.py on build

[santisoler]

Move setuptools-scm to requirements-dev.txt. Automatically create
the version string on build and store it in harmonica/_version.py.
Remove the need to run pgk_resources to get the version number. Invert
MANIFEST.in by specifying what should be ignored because setuptools-scm
includes everything by default.

Related to fatiando/maintenance#4

Inspired on fatiando/pooch#235

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

@leouieda We are still using pkg_resources on datasets/sample_data.py to load the registry file. Should we leave that as it is or we would like to change that too?

[leouieda]

@santisoler we would still need it for the registry so leave it as is. I'm leaning towards defining the registry as a Python dict in the module instead of the external file. It's no harder to do and easier to maintain since there is no messing with setup.py and MANIFEST.in.

[leouieda]

👍🏽 now we just pray that this doesn't break everything. Would be good to download and inspect the test PyPI package after this is merged.