Return Python filename with ``__version__`` marker, if configured.

        Enable this by adding a ``python-file-with-version`` option::

            [zest.releaser]
            python-file-with-version = reinout/maurits.py

        Return None when nothing has been configured.

def python_file_with_version(self):
        """Return Python filename with ``__version__`` marker, if configured.

        Enable this by adding a ``python-file-with-version`` option::

            [zest.releaser]
            python-file-with-version = reinout/maurits.py

        Return None when nothing has been configured.

        """
        default = None
        if self.config is None:
            return default
        try:
            result = self.config.get('zest.releaser',
                                     'python-file-with-version')
        except (NoSectionError, NoOptionError, ValueError):
            return default
        return result