'Initialise an instance.
:param scheme: Because locators look for most recent versions, they
need to know the version scheme to use. This specifies
the current PEP-recommended scheme - use ``\'legacy\'``
if you need to support existing distributions on PyPI.'

def __init__(self, scheme='default'):

    self._cache = {}
    self.scheme = scheme
    self.opener = build_opener(RedirectHandler())
    self.matcher = None
