client: make _monkey_patch_oauth2client_locked_file run on Py2/3
client: make _monkey_patch_oauth2client_locked_file run on Py2/3

non instantiated class function does not have __func__ in python3.

Bug: 1010816
Change-Id: Iab5fc53561acd302e38abacab3d99d0ae1e46f4b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/1875967
Commit-Queue: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Junji Watanabe <aaef61ef98ffea505db831f310be2a0eb8492537@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

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