Fixes #23837: Replace list.pop(0) by deque.popleft() in dfs function in migrations

from __future__ import unicode_literals

from collections import deque

from django.db.migrations.state import ProjectState

from django.utils.datastructures import OrderedSet

    def dfs(self, start, get_children):

        """

        Dynamic programming based depth first search, for finding dependencies.

        Iterative depth first search, for finding dependencies.

        """

        visited = []

        visited = deque()

        visited.append(start)

        path = [start]

        stack = sorted(get_children(start))

        stack = deque(sorted(get_children(start)))

        while stack:

            node = stack.pop(0)

            node = stack.popleft()

            if node in path:

                raise CircularDependencyError()

            path.append(node)

            visited.insert(0, node)

            children = sorted(get_children(node))

            visited.appendleft(node)

            children = sorted(get_children(node), reverse=True)

            # reverse sorting is needed because prepending using deque.extendleft

            # also effectively reverses values

            if not children:

                path = []

            stack = children + stack

            stack.extendleft(children)

        return list(OrderedSet(visited))