This reverts commit 02927937.

Reason for revert: https://crbug.com/1175200

Original change's description:
> Bump python2 to 2.7.18
>
> That version was pushed to bleeding_edge in
> https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2861642
>
> Bug: 1124119
> Change-Id: Id4ab5cb599dc85f3da7784163ddd46b2cd7c7980
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2867872
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>

Bug: 1124119
Change-Id: I70124c502c84713e2bbb809793353c74e2e40645
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2872006
Auto-Submit: Nico Weber <thakis@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>