Migrate check_elf_file.py to python3

Also, create a python_binary_host so that it runs using the hermetic
python toolchain.

Test: m check-elf-files
Bug: 203436762
Change-Id: I964342a27f6b6c9dcdbbe910d4dc6c9ec00c2213

tools/check_elf_file.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2019 The Android Open Source Project
 #
@@ -196,11 +196,7 @@ class ELFParser(object):
   def _read_llvm_readobj(cls, elf_file_path, header, llvm_readobj):
     """Run llvm-readobj and parse the output."""
     cmd = [llvm_readobj, '--dynamic-table', '--dyn-symbols', elf_file_path]
-    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
-    out, _ = proc.communicate()
-    rc = proc.returncode
-    if rc != 0:
-      raise subprocess.CalledProcessError(rc, cmd, out)
+    out = subprocess.check_output(cmd, text=True)
     lines = out.splitlines()
     return cls._parse_llvm_readobj(elf_file_path, header, lines)
 
@@ -467,7 +463,7 @@ class Checker(object):
     """Check whether all undefined symbols are resolved to a definition."""
     all_elf_files = [self._file_under_test] + self._shared_libs
     missing_symbols = []
-    for sym, imported_vers in self._file_under_test.imported.iteritems():
+    for sym, imported_vers in self._file_under_test.imported.items():
       for imported_ver in imported_vers:
         lib = self._find_symbol_from_libs(all_elf_files, sym, imported_ver)
         if not lib: