Switch from versioneer to setuptools-scm

[weiji14]

Description of proposed changes

Replace versioneer added in 3683f37 with the more modern setuptools-scm.

This PR mostly follows psf/black#1008, and fatiando/boule#61:

• Remove all instances of versioneer and pygmt/_version.py. There is a _version.py file at the top-level which is git-ignored that contains a single line like __version__ = "0.2.2.dev3+g355d85f1"
• Update our Github Actions workflows that mentioned versioneer

What this PR doesn't do:

• Place configuration under pyproject.toml instead of setup.py (see best practice recommended at https://github.com/pypa/setuptools_scm/tree/v4.1.2#pyprojecttoml-usage). Maybe do it when we remove setup.py and replace it entirely with pyproject.toml.

Fixes #671

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.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[weiji14]

I wonder if writing to a _version.py file is actually needed here.

[seisman]

https://github.com/fatiando/boule doesn't write the file.

[weiji14]

Ok, I'll remove this line then (and don't add _version.py to .gitignore too).

[seisman]

Looks good.