support nested lists

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right now python-docstring-mode thinks

- this
   - and this

are bullets at the same indentation level and reflows them accordingly. this should be fixed.

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Not sure exactly when this was fixed, but this:

def fun(a):
    """
    - this
      - and this
    """

now correctly reflows to this:

def fun(a):
    """
        - this

            - and this
    """