Explicitly exclude unnecessary files in source distributions (#999)
Explicitly exclude unnecessary files in source distributions (#999)

Running `python setup.py sdist` can generate source distributions.

When packaging, setuptools_scm includes all files tracked by git.

This PR explicitly excludes unnecessary files and directories 
from the source distributions.

Co-authored-by: Wei Ji <9490087a8b98c7de12813b2e194ef86ac3043a88@users.noreply.github.com>
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python