Remove the `<BLANKLINE>` from doctests when we are able

[drbenvincent]

• The PR Fix failing doctests #330 fixes some failing doctests. There were a couple of things going on there, but one of the main reasons was because (probably) the progress bar was outputting empty lines.
• Turns out that this is the case and @tomicapretto is already on it with Implement CustomProgress that does not output empty divs when disabled pymc-devs/pymc#7290
• Once this PR is merged and the changes filter through into a new PyMC release, then we can go back to the docstrings and remove all the <BLANKLINE> instances.