Set py36 to false in Black defaults

As Python2 is still supported then the Python 3.6+ option, py36, must be
set to false in the default configuration as it will:
> will put trailing commas in function signatures and calls also after
> *args and **kwargs.

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.black]
 line-length = 88
-py36 = true
+py36 = false  # Python 2 is still supported
 skip-string-normalization = true
 skip-numeric-underscore-normalization = true  # Python 2 is still supported
 include = '\.pyi?$'

tests/test_notebooks.py

@@ -27,12 +27,12 @@ def test_notebooks(tmpdir):
     pm.execute_notebook(
         'docs/examples/notebooks/multichannel-coupled-histo.ipynb',
         parameters={'validation_datadir': 'validation/data'},
-        **common_kwargs,
+        **common_kwargs
     )
     pm.execute_notebook(
         'docs/examples/notebooks/multiBinPois.ipynb',
         parameters={'validation_datadir': 'validation/data'},
-        **common_kwargs,
+        **common_kwargs
     )
 
     nb = pm.read_notebook(str(outputnb))