Use python3 on macOS (#15102)

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

Closes #15045.

PiperOrigin-RevId: 436548693
(cherry picked from commit 3785677cc84fc4024fda85575c05efbde5d512fc)

Co-authored-by: Oscar Bonilla <6f6231@gmail.com>
diff --git a/tools/objc/libtool.sh b/tools/objc/libtool.sh
index 94219e2..0d8f643 100755
--- a/tools/objc/libtool.sh
+++ b/tools/objc/libtool.sh
@@ -62,9 +62,9 @@
   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=()