Fix an upcoming pytype error in absltest.

I'm working on an improvement to pytype's type-checking of function type
comments, which exposes an annotation error in absltest.
_TempFile._open() was annotated as returning Union[TextIO, BinaryIO] but is
used as:

def open_text(...) -> ContextManager[TextIO]:
  ...
  return self._open(...)

which will cause pytype to complain about open_text returning bytes when text
is expected. The proper way to fix this issue would be to use typing.overload
and typing.Literal to define multiple signatures for _open(), but pytype
doesn't fully support Literal yet, so I've added type comments with the
right types in open_text() and open_bytes().

This change also fixes a mistake in the return annotation on _open() and
alphabetically orders all typing imports.

PiperOrigin-RevId: 283783117
Change-Id: I4d78a9eb1c98218e62a9b0662931213f7c5c1668

absl/testing/absltest.py

@@ -70,7 +70,7 @@
 try:
   # pylint: disable=unused-import
   import typing
-  from typing import AnyStr, Callable, Text, Optional, ContextManager, TextIO, BinaryIO, Union, Type, Tuple, Any, MutableSequence, Sequence, Mapping, MutableMapping, IO, List
+  from typing import Any, AnyStr, BinaryIO, Callable, ContextManager, IO, Iterator, List, Mapping, MutableMapping, MutableSequence, Optional, Sequence, Text, TextIO, Tuple, Type, Union
   # pylint: enable=unused-import
 except ImportError:
   pass
@@ -484,7 +484,8 @@ def open_text(self, mode='rt', encoding='utf8', errors='strict'):
                        'file in text mode'.format(mode))
     if 't' not in mode:
       mode += 't'
-    return self._open(mode, encoding, errors)
+    cm = self._open(mode, encoding, errors)  # type: ContextManager[TextIO]
+    return cm
 
   def open_bytes(self, mode='rb'):
     # type: (Text) -> ContextManager[BinaryIO]
@@ -505,11 +506,15 @@ def open_bytes(self, mode='rb'):
                        'file in binary mode'.format(mode))
     if 'b' not in mode:
       mode += 'b'
-    return self._open(mode, encoding=None, errors=None)
+    cm = self._open(mode, encoding=None, errors=None)  # type: ContextManager[BinaryIO]
+    return cm
 
+  # TODO(b/123775699): Once pytype supports typing.Literal, use overload and
+  # Literal to express more precise return types and remove the type comments in
+  # open_text and open_bytes.
   @contextlib.contextmanager
   def _open(self, mode, encoding='utf8', errors='strict'):
-    # type: (Text, Text, Text) -> Union[TextIO, BinaryIO]
+    # type: (Text, Text, Text) -> Iterator[Union[IO[Text], IO[bytes]]]
     with io.open(
         self.full_path, mode=mode, encoding=encoding, errors=errors) as fp:
       yield fp