The code-block in generated julia notebooks are executable

[jlperla]

As an example, in the getting_started lecture, there is a cell with {code-block} julia and not code-cell.

It is shown correctly on the HTML and not-executed, but the generated notebook has it as a code cell (rather than as just markdown). While that example is simple and we could just convert it to a none cell to prevent execution, in many places it is not so innocuous. In particular, in the testing lecture, etc.

@mmcky FYI: not a blocker for release, but this will be important.

[mmcky]

@jlperla thanks for opening this issue. I have opened this issue QuantEcon/sphinx-tojupyter#36