[roll] Roll fuchsia [python] python3->python3.8

Today python3 == python3.8, but this is about to change.
Many scripts assume python3.8.
All scripts that specify python3 work with python3.8.
Explicit is better than implicit.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/511039
Original-Revision: b6b861f831071ca693af02d89975a29bbf7a0c98
GitOrigin-RevId: c0322db437c837c61d49cd56b71287182b1084c1
Change-Id: Ia0e0e4b00266f7551b28adf00c52319ac2c49dc3
diff --git a/stem b/stem
index 1d4d9a9..23a6bf9 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="c364cb4883cf6b4dd1928c98392c2c687151a3f3"/>
+             revision="b6b861f831071ca693af02d89975a29bbf7a0c98"/>
   </projects>
   <hooks>
     <hook name="install-environment"