Update hacking `env_setup` to prefer Python 3 (#75213)

* env_setup - Prefer Python3

Prefer the 'python3' executable over 'python'.

* Add shebang for syntax detection

While the script isn't executable and we recommend sourcing it, having a shebang allows
variuos text editors to detect that it is a shell script and apply proper syntax highlighting.

hacking/env-setup

@@ -1,4 +1,4 @@
-
+#!/bin/sh
 # usage: source hacking/env-setup [-q]
 #    modifies environment for running Ansible from checkout
 
@@ -22,7 +22,7 @@ prepend_path()
 PYTHONPATH=${PYTHONPATH-""}
 PATH=${PATH-""}
 MANPATH=${MANPATH-$(manpath)}
-PYTHON=$(command -v python || command -v python3)
+PYTHON=$(command -v python3 || command -v python)
 PYTHON_BIN=${PYTHON_BIN-$PYTHON}
 
 verbosity=${1-info} # Defaults to `info' if unspecified

hacking/env-setup.fish

@@ -47,10 +47,10 @@ end
 
 # Set PYTHON_BIN
 if not set -q PYTHON_BIN
-    if test (which python)
-        set -gx PYTHON_BIN (which python)
-    else if test (which python3)
+    if test (which python3)
         set -gx PYTHON_BIN (which python3)
+    else if test (which python)
+        set -gx PYTHON_BIN (which python)
     else
         echo "No valid Python found"
         exit 1