cpython3 3.8.10.chromium.20 to depot_tools stable.

Bug: 1220743
Change-Id: I72dac4c01f21670cb61edf19f73fa3b89b8d78e0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3038232
Auto-Submit: Brian Ryner <bryner@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

bootstrap/manifest.txt

@@ -21,7 +21,7 @@ $VerifiedPlatform windows-amd64 linux-amd64 mac-amd64
 infra/3pp/tools/cpython/${platform} version:2@2.7.18.chromium.39
 
 @Subdir python3
-infra/3pp/tools/cpython3/${platform} version:2@3.8.10.chromium.19
+infra/3pp/tools/cpython3/${platform} version:2@3.8.10.chromium.20
 
 # TODO(https://crbug.com/1184766): Remove `=amd64` once mac/arm64 git exists
 @Subdir git