Switch to explicit gitinclude pattern

.gitignore

-build/
-__pycache__/
-*.egg-info/
-.ipynb_checkpoints
-.dist/
-.coverage
-.mypy_cache/
-public/
-.idea/
-playground.py
-.vscode/
-cache.json
-docs/_build/
-report.md
+# Ignore every file and directory
+*
+
+# Except
+!docs/*
+!docs/_static/*
+!src
+!src/coscine
+!src/coscine/*.py
+!src/coscine/py.typed
+!tests
+!tests/*.py
+!.gitignore
+!.gitlab-ci.yml
+!LICENSE.txt
+!README.md
+!requirements.txt
+!setup.py