Include missing files in sdist

[thatch]

Include missing files in sdist for #1

Tested with

++ mktemp -d
+ D=/tmp/tmp.Y3byXqgISZ
+ trap 'rm -rf /tmp/tmp.Y3byXqgISZ' EXIT
+ python -m venv /tmp/tmp.Y3byXqgISZ
+ python setup.py sdist -d /tmp/tmp.Y3byXqgISZ
running sdist
running egg_info
writing jspline.egg-info/PKG-INFO
writing dependency_links to jspline.egg-info/dependency_links.txt
writing top-level names to jspline.egg-info/top_level.txt
reading manifest file 'jspline.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'jspline.egg-info/SOURCES.txt'
running check
creating jspline-1.0.3
creating jspline-1.0.3/jspline
creating jspline-1.0.3/jspline.egg-info
copying files to jspline-1.0.3...
copying HISTORY.md -> jspline-1.0.3
copying MANIFEST.in -> jspline-1.0.3
copying README.md -> jspline-1.0.3
copying setup.py -> jspline-1.0.3
copying jspline/__init__.py -> jspline-1.0.3/jspline
copying jspline/jspline.py -> jspline-1.0.3/jspline
copying jspline.egg-info/PKG-INFO -> jspline-1.0.3/jspline.egg-info
copying jspline.egg-info/SOURCES.txt -> jspline-1.0.3/jspline.egg-info
copying jspline.egg-info/dependency_links.txt -> jspline-1.0.3/jspline.egg-info
copying jspline.egg-info/top_level.txt -> jspline-1.0.3/jspline.egg-info
Writing jspline-1.0.3/setup.cfg
Creating tar archive
removing 'jspline-1.0.3' (and everything under it)
+ cd /
+ /tmp/tmp.Y3byXqgISZ/bin/pip install /tmp/tmp.Y3byXqgISZ/jspline-1.0.3.tar.gz
Processing /tmp/tmp.Y3byXqgISZ/jspline-1.0.3.tar.gz
Installing collected packages: jspline
  Running setup.py install for jspline: started
    Running setup.py install for jspline: finished with status 'done'
Successfully installed jspline-1.0.3
WARNING: You are using pip version 19.2.3, however version 20.1.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
+ echo Success
Success
++ rm -rf /tmp/tmp.Y3byXqgISZ