Return highest minor of a list of stable (semantic) versions.

    Example:
        >>> python_versions = [
        ...     '2.6.9', '2.7.14', '3.3.7', '3.4.8', '3.5.5', '3.6.4']
        >>> highest_minor(python_versions)
        '3.6'

def highest_minor(python_versions):
    '''Return highest minor of a list of stable (semantic) versions.

    Example:
        >>> python_versions = [
        ...     '2.6.9', '2.7.14', '3.3.7', '3.4.8', '3.5.5', '3.6.4']
        >>> highest_minor(python_versions)
        '3.6'

    '''
    highest = python_versions[-1]
    major, minor, patch = highest.split('.', 2)
    return flo('{major}.{minor}')