Update git submodules

* Update keystone from branch 'master'
  to 39ac6e70d7525c0f321fb4b56ffff0f199cac8a4
  - Merge "Drop flake8-docstrings"
  - Drop flake8-docstrings

    It depends on pydocstyle which was deprecated and archived. See [1]
    where it was indicated that the plugin is also being dead.

    [1] https://github.com/PyCQA/flake8-docstrings/issues/68

    Change-Id: I5f017a9cf0ae74fef998b1658fc3b8be8cde8b51
    Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

diff --git a/keystone b/keystone
index 19e1b3fc91d960de232dbc4d356e681ad12317d1..39ac6e70d7525c0f321fb4b56ffff0f199cac8a4 160000 (submodule)
--- a/keystone
+++ b/keystone
@@ -1 +1 @@
-Subproject commit 19e1b3fc91d960de232dbc4d356e681ad12317d1
+Subproject commit 39ac6e70d7525c0f321fb4b56ffff0f199cac8a4