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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def create_releasenotes(project_dir=os.curdir, bugtracker_url=''):
 """ Creates the release notes file, if not in a package. Args: project_dir(str):
 Path to the git repo of the project. bugtracker_url(str):
 Url to the bug tracker for the issues. Returns: None Raises: RuntimeError: If the release notes could not be retrieved """

    pkg_info_file = os.path.join(project_dir, 'PKG-INFO')
    if os.path.exists(pkg_info_file):
        return

    with open('RELEASE_NOTES', 'wb') as releasenotes_fd:
        releasenotes_fd.write(
            get_releasenotes(
                project_dir=project_dir,
                bugtracker_url=bugtracker_url,
            ).encode('utf-8') + b'\n'
        )