Bump python2 to 2.7.18 again, and pull it on mac/arm

This time, the mac/intel binary was built with an older macOS SDK
which makes the runtime believe that the macOS version on Big Sur is
10.16 instead of 11.0, which should work around the Big Sur issue we had
in the last attempt.

This version was pushed to bleeding_edge in
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2876311

Bug: 1124119,1103236,1184768,1175200
Change-Id: I8614d6f22704d46bfa78b81c8e4b9ff7e6668487
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2880559
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>

bootstrap/manifest.txt

@@ -17,9 +17,8 @@
 
 $VerifiedPlatform windows-amd64 linux-amd64 mac-amd64
 
-# TODO(https://crbug.com/1184768): Remove `=amd64` once mac/arm64 python exists
 @Subdir python
-infra/3pp/tools/cpython/${os}-${arch=amd64} version:2.7.17.chromium.22
+infra/3pp/tools/cpython/${platform} version:2@2.7.18.chromium.35
 
 @Subdir python3
 infra/3pp/tools/cpython3/${platform} version:2@3.8.9.chromium.14