simplify MANIFEST.in file
It seems that this file is a copy/paste from another project and is not
relevant for this repository.
-exclude .gitignore
-exclude .coverage
-exclude .travis.yml
-include README.rst
-include setup.cfg
-prune .cache
-prune .git
-prune build
-prune dist
-recursive-exclude *.egg-info *
-recursive-include tests *
+include setup.cfg README.md
include substra/run_local_materials *
+
+recursive-include tests *
