Use explicit python_version: "3.8".

Wheels are only supposed to work with a specific minor version of python.

Bug:b/275218507
Change-Id: I0bc89ff6f64a4aaaba863615703535f6cfa1ad05
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4372601
Reviewed-by: Katherine Threlkeld <kathrelkeld@chromium.org>
Tested-by: Chenlin Fan <fancl@chromium.org>
Commit-Queue: Chenlin Fan <fancl@chromium.org>
Auto-Submit: Chenlin Fan <fancl@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4411617
Tested-by: Shou-Chieh Hsu <shouchieh@chromium.org>
Commit-Queue: Derek Huang <derekhuang@google.com>
Reviewed-by: Shou-Chieh Hsu <shouchieh@chromium.org>
diff --git a/src/chromiumos/test/python/src/tools/test/.vpython b/src/chromiumos/test/python/src/tools/test/.vpython
new file mode 100644
index 0000000..06824af
--- /dev/null
+++ b/src/chromiumos/test/python/src/tools/test/.vpython
@@ -0,0 +1 @@
+python_version: "3.8"