Running python setup.py sdist will build a PyGMT source distribution in the dist directory (e.g., dist/pygmt-0.3.0.tar.gz.

The tarball contains the PyGMT source codes. Below is the list of top-level files and directories:

.codecov.yml
.github
.gitignore
.pylintrc
AUTHORS.md
AUTHORSHIP.md
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE.txt
MAINTENANCE.md
MANIFEST.in
Makefile
PKG-INFO
README.rst
doc
environment.yml
examples
package.json
pygmt
pygmt.egg-info
pyproject.toml
requirements-dev.txt
requirements.txt
setup.cfg
setup.py
vercel.json

I'm not familiar with Python source distributions, but some files seem unnecessary (e.g., vercel.json, package.json, .github).

Reading the Python package guide, I thought most plaintext files won't be added into the source distribution, unless explicitly specified in the MANIFEST.in file. Perhaps my understanding is wrong.