Scope `six` import in utils/tools.py
Scope `six` import in utils/tools.py

Normally, utils/tools.py is imported, and then third_party imports are
allowed. Since six is a third_party import, having utils/tools.py take
a dependency on it creates a potential conflict. Limiting this import,
by putting it inside a function, avoids this issue.

Change-Id: Id81094175e64a524ee623e71df1163d57dfe651b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2280404
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py