def __init__(self, dependency_name, is_upstream=False, optional=False):
self.dependency_name = dependency_name<EOL>self.is_upstream = is_upstream<EOL>self.optional = optional<EOL>
Constructor for `Extension`.

Args:
    dependency_name: str, see `ExtDependency.dependency_name`
    is_upstream: bool, see `ExtDependency.is_upstream`