GitLab

Our documentation builder calls external scripts to produce Graphviz
dot code representing pipelines.

On the one hand, under both Python2 and Python3 the subprocess output
is a bytestring. On the other, some internals Sphinx Graphviz module
expects dot code to have string representation, which is NOT the same
as bytestrings under Python3. As a result, when running under Python3
the Graphviz module fails to render dot code due to being unable to
concatenate a string with a bytestring.

Now subprocess output is decoded to string, using the default encoding
of the given Python version. This makes the Graphviz module happy.

Seems to work fine under both Python2 and Python3 now. Switching to
non-binary mode was needed due to Python3 strings being Unicode by
default.

Finally added the copyright statement on all source code files and the test to ensure it is kept up to date