setup: Make check-manifest happy

Before this patch it was complaining:

    (xlte3.venv) kirr@deca:~/src/wendelin/xlte$ check-manifest
    lists of files in version control and sdist do not match!
    missing from sdist:
      demo/kpidemo.ipynb
      demo/kpidemo.py
      xlte.py
    suggested MANIFEST.in rules:
      include *.py
      recursive-include demo *.ipynb
      recursive-include demo *.py

Noticed during !2 review.

MANIFEST.in

 include COPYING README.rst CHANGELOG.rst
+include *.py
+recursive-include amari	*.py
+recursive-include demo	*.py *.ipynb
+recursive-exclude */.ipynb_checkpoints *