Use python3 on macOS

macOS 12.3 removed `/usr/bin/python2.7` and `/usr/bin/python`. We need to use `python3` now.

Closes #15045.

PiperOrigin-RevId: 436548693

Author: ob

tools/objc/libtool.sh

@@ -62,9 +62,9 @@ function hash_objfile() {
   echo "$SYMLINK_NAME"
 }
 
-python_executable=/usr/bin/python2.7
+python_executable=/usr/bin/python3
 if [[ ! -x "$python_executable" ]]; then
-  python_executable=python
+  python_executable=python3
 fi
 
 ARGS=()