In Python 3 byestrings and normal strings can't be concatenated.
To fix this we buffer as bytestrings in the Annotated wrapper.
We can't decode to a string because the output might come byte-by-byte, so it doesn't work with Unicode characters like ✔.

Also had to update gclient_test.py, where double-wrapping stdout with Annotated caused made output not work and include_zero=True working caused other unintended side-effects.

Example error from "fetch chromium":
Traceback (most recent call last):
  File "C:\Google\depot_tools\gclient_scm.py", line 1045, in _Clone
    self._Run(clone_cmd, options, cwd=self._root_dir, retry=True,
  File "C:\Google\depot_tools\gclient_scm.py", line 1370, in _Run
    gclient_utils.CheckCallAndFilter(cmd, env=env, **kwargs)
  File "C:\Google\depot_tools\gclient_utils.py", line 583, in CheckCallAndFilter
    show_header_if_necessary(needs_header, attempt)
  File "C:\Google\depot_tools\gclient_utils.py", line 533, in show_header_if_necessary
    stdout_write(header.encode())
  File "C:\Google\depot_tools\gclient_utils.py", line 391, in write
    obj[0] += out
TypeError: can only concatenate str (not "bytes") to str

Bug: 984182
Change-Id: If7037d30e9faf524f2405258281f6e6cd0bcdcae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1778745
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Raul Tambre <raul@tambre.ee>