api_checker: Drop support for Python 2

This converts the `utils/api_checker` scripts to support Python 3 only.
It removes all `from __future__` imports, and switches any `python`
shebangs to `python3`.

Author: jorng

utils/api_checker/sdk-module-lists/infer-imports.py

@@ -1,6 +1,4 @@
-#!/usr/bin/env python -u
-
-from __future__ import print_function
+#!/usr/bin/env python3 -u
 
 import os
 import sys

utils/api_checker/swift-api-checker.py

@@ -1,7 +1,5 @@
 #!/usr/bin/env python3
 
-from __future__ import print_function
-
 import argparse
 import os
 import subprocess