Fix wrong version numbers for PyPI releases

[santisoler]

For test releases to TestPyPI we have to edit the setuptools_scm
configuration to produce valid version numbers. But it turns out that
this breaks the number for actual releases. To fix it, only edit the
configuration on non-release builds.

Issue raised on fatiando/verde#317

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.