build: Revise MANIFEST.in strategy

[matthewfeickert]

The contents of the heputils sdists are distributing more things that is intentional. Only the source code should be distributed, but at the moment the whole repository is being distributed instead.

$ tar -ztvf /heputils-0.0.7.dev8.tar.gz 
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/
-rw-r--r-- runner/docker   137 2021-07-31 02:01 heputils-0.0.7.dev8/.bumpversion.cfg
-rw-r--r-- runner/docker   222 2021-07-31 02:01 heputils-0.0.7.dev8/.coveragerc
-rw-r--r-- runner/docker   104 2021-07-31 02:01 heputils-0.0.7.dev8/.flake8
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/.github/
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/.github/workflows/
-rw-r--r-- runner/docker  1279 2021-07-31 02:01 heputils-0.0.7.dev8/.github/workflows/ci.yml
-rw-r--r-- runner/docker   616 2021-07-31 02:01 heputils-0.0.7.dev8/.github/workflows/lint.yml
-rw-r--r-- runner/docker  2143 2021-07-31 02:01 heputils-0.0.7.dev8/.github/workflows/publish-package.yml
-rw-r--r-- runner/docker  1831 2021-07-31 02:01 heputils-0.0.7.dev8/.gitignore
-rw-r--r-- runner/docker  1176 2021-07-31 02:01 heputils-0.0.7.dev8/.pre-commit-config.yaml
-rw-r--r-- runner/docker  3221 2021-07-31 02:01 heputils-0.0.7.dev8/CODE_OF_CONDUCT.md
-rw-r--r-- runner/docker  1524 2021-07-31 02:01 heputils-0.0.7.dev8/LICENSE
-rw-r--r-- runner/docker    65 2021-07-31 02:01 heputils-0.0.7.dev8/MANIFEST.in
-rw-r--r-- runner/docker  4700 2021-07-31 02:01 heputils-0.0.7.dev8/PKG-INFO
-rw-r--r-- runner/docker  3516 2021-07-31 02:01 heputils-0.0.7.dev8/README.md
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/binder/
-rw-r--r-- runner/docker    19 2021-07-31 02:01 heputils-0.0.7.dev8/binder/apt.txt
-rw-r--r-- runner/docker   683 2021-07-31 02:01 heputils-0.0.7.dev8/binder/postBuild
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/examples/
-rw-r--r-- runner/docker  8153 2021-07-31 02:01 heputils-0.0.7.dev8/examples/dev-example.ipynb
-rw-r--r-- runner/docker   726 2021-07-31 02:01 heputils-0.0.7.dev8/pyproject.toml
-rw-r--r-- runner/docker   178 2021-07-31 02:01 heputils-0.0.7.dev8/pytest.ini
-rw-r--r-- runner/docker  1453 2021-07-31 02:01 heputils-0.0.7.dev8/setup.cfg
-rw-r--r-- runner/docker   664 2021-07-31 02:01 heputils-0.0.7.dev8/setup.py
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/src/
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/
-rw-r--r-- runner/docker   197 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/__init__.py
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/cli/
-rw-r--r-- runner/docker   107 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/cli/__init__.py
-rw-r--r-- runner/docker   270 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/cli/cli.py
-rw-r--r-- runner/docker  1729 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/convert.py
-rw-r--r-- runner/docker 17304 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/plot.py
-rw-r--r-- runner/docker  2214 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/utils.py
-rw-r--r-- runner/docker   174 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils/version.py
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/
-rw-r--r-- runner/docker  4700 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/PKG-INFO
-rw-r--r-- runner/docker   763 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/SOURCES.txt
-rw-r--r-- runner/docker     1 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/dependency_links.txt
-rw-r--r-- runner/docker    47 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/entry_points.txt
-rw-r--r-- runner/docker   484 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/requires.txt
-rw-r--r-- runner/docker     9 2021-07-31 02:01 heputils-0.0.7.dev8/src/heputils.egg-info/top_level.txt
drwxr-xr-x runner/docker     0 2021-07-31 02:01 heputils-0.0.7.dev8/tests/
-rw-r--r-- runner/docker  6364 2021-07-31 02:01 heputils-0.0.7.dev8/tests/example_files.py
-rw-r--r-- runner/docker   400 2021-07-31 02:01 heputils-0.0.7.dev8/tests/test_cli.py
-rw-r--r-- runner/docker    57 2021-07-31 02:01 heputils-0.0.7.dev8/tests/test_import.py

To fix this, use the same strategy as scikit-hep/pyhf#1449.