convert `.version` to property

[classabbyamp]

This also makes for better documentation.

Steps:

1. s/self.version/self._version
2. remove version from class docstring
3. add this:

    @property
    def version(self) -> str:
        """The version/date of the current BigCTY data.

        :getter: Returns version in ``YYYYMMDD`` format
        :type: str
        """
        return self._version