Returns a Basic Auth string.

def _basic_auth_str(username, password):
    """Returns a Basic Auth string."""

    # "I want us to put a big-ol' comment on top of it that
    # says that this behaviour is dumb but we need to preserve
    # it because people are relying on it."
    #    - Lukasa
    #
    # These are here solely to maintain backwards compatibility
    # for things like ints. This will be removed in 3.0.0.
    if not isinstance(username, basestring):
        warnings.warn(
            "Non-string usernames will no longer be supported in Requests "
            "3.0.0. Please convert the object you've passed in ({!r}) to "
            "a string or bytes object in the near future to avoid "
            "problems.".format(username),
            category=DeprecationWarning,
        )
        username = str(username)

    if not isinstance(password, basestring):
        warnings.warn(
            "Non-string passwords will no longer be supported in Requests "
            "3.0.0. Please convert the object you've passed in ({!r}) to "
            "a string or bytes object in the near future to avoid "
            "problems.".format(password),
            category=DeprecationWarning,
        )
        password = str(password)
    # -- End Removal --

    if isinstance(username, str):
        username = username.encode('latin1')

    if isinstance(password, str):
        password = password.encode('latin1')

    authstr = 'Basic ' + to_native_string(
        b64encode(b':'.join((username, password))).strip()
    )

    return authstr