pass the context to the doctest format

[keewis]

The doctest format can appear in both python code and other formats, but docstrings need special attention: naive reformatting may cause triple-quoted strings to conflict with the docstring quotes.

To solve this, it would be good to somehow detect the type of docstring quote we're in instead of trying to reproduce exactly the quote we had before. I'm guessing that's really difficult to do using a line parser, though.