releasetools: Make common.ZipWriteStr Python 3 compatible.

Python 2 and 3 behave differently when calling ZipFile.writestr() with
zinfo.external_attr being 0. Python 3 uses `0o600 << 16` as the value
for such a case (since
18ee29d0b8),
which seems to make more sense. Otherwise the entry will end up with
0o000 as the permission bits. This CL updates common.ZipWriteStr to
follow the logic in Python 3, in order to get consistent behavior
between using the two versions.

Bug: 131631303
Test: `python -m unittest test_common.CommonZipTest`
Test: `python3 -m unittest test_common.CommonZipTest`
Change-Id: If8429855d922ef1ad76591f703215a0ce5089f0f

tools/releasetools/common.py

@@ -863,7 +863,7 @@ def GetUserImage(which, tmpdir, input_zip,
     A Image object. If it is a sparse image and reset_file_map is False, the
     image will have file_map info loaded.
   """
-  if info_dict == None:
+  if info_dict is None:
     info_dict = LoadInfoDict(input_zip)
 
   is_sparse = info_dict.get("extfs_sparse_flag")
@@ -1568,6 +1568,15 @@ def ZipWriteStr(zip_file, zinfo_or_arcname, data, perms=None,
       perms = 0o100644
   else:
     zinfo = zinfo_or_arcname
+    # Python 2 and 3 behave differently when calling ZipFile.writestr() with
+    # zinfo.external_attr being 0. Python 3 uses `0o600 << 16` as the value for
+    # such a case (since
+    # https://github.com/python/cpython/commit/18ee29d0b870caddc0806916ca2c823254f1a1f9),
+    # which seems to make more sense. Otherwise the entry will have 0o000 as the
+    # permission bits. We follow the logic in Python 3 to get consistent
+    # behavior between using the two versions.
+    if not zinfo.external_attr:
+      zinfo.external_attr = 0o600 << 16
 
   # If compress_type is given, it overrides the value in zinfo.
   if compress_type is not None:
@@ -1600,7 +1609,7 @@ def ZipDelete(zip_filename, entries):
   Raises:
     AssertionError: In case of non-zero return from 'zip'.
   """
-  if isinstance(entries, basestring):
+  if isinstance(entries, str):
     entries = [entries]
   cmd = ["zip", "-d", zip_filename] + entries
   RunAndCheckOutput(cmd)