Broken docs examples

[BSchilperoort]

The examples in, e.g., calendar_shifter are broken due to a missing empty line between the text and example code:

    Example:
        Shift a calendar by a given dateoffset.
        >>> import lilio

Should be:

    Example:
        Shift a calendar by a given dateoffset.

        >>> import lilio

doctest does not catch this mistake, and pydocstyle does not either (at least using the default settings).

It would be nice if we could automatically detect these mistakes with a linter.

[BSchilperoort]

Fixed by #16