launcher: Set shebang to python3

Some (most?) Linux repos don't have /usr/bin/python, unless
python-is-python3 is installed. While package owners can adjust shebang,
we have seen an increase in number of bugs filed as extra steps are
required.

Per PEP-0394, python3 is acceptable and should be available if python3
is supported. We no longer support python2, and repo no longer works
with python2, so this change makes that explicit.

Bug: 40014585
Change-Id: I9aed90fd470ef601bd33bd596af3df69da69ef5d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/407497
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Jason Chang <jasonnc@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Максим Паймушкин <maxim.paymushkin@gmail.com>
Tested-by: Josip Sokcevic <sokcevic@google.com>

repo

diff --git a/repo b/repo
index b598c32..adcfb6e 100755
--- a/repo
+++ b/repo
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2008 The Android Open Source Project
 #
@@ -124,7 +124,7 @@
 BUG_URL = "https://issues.gerritcodereview.com/issues/new?component=1370071"
 
 # increment this whenever we make important changes to this script
-VERSION = (2, 40)
+VERSION = (2, 42)
 
 # increment this if the MAINTAINER_KEYS block is modified
 KEYRING_VERSION = (2, 3)