Convert inline python code to markdown

[machow]

It looks like rst directives like :py:inherit.aes=True (e.g. in geom_abline) can be converted to markdown using this syntax:

• `a = "abc"`{.python}

[machow]

We could also add a filter, that sets the python class on all inline code blocks (that don't have a class set)

edit: hassan very wisely pointed out that we won't be able to tell apart literal text (using inline blocks) from python code in a filter.