Merge branch 'release/1.6.8'

pybis/src/python/.gitignore

pybis/src/python/notebooks/pybis demo.ipynb

pybis/src/python/pybis/CHANGELOG.md

pybis/src/python/pybis/__init__.py

pybis/src/python/pybis/attribute.py

pybis/src/python/pybis/dataset.py

pybis/src/python/pybis/definitions.py

pybis/src/python/pybis/pybis.py

pybis/src/python/pybis/sample.py

pybis/src/python/setup.py

pybis/src/python/tests/test_sample.py