Make _preserve_encoding in platform_utils_win32 compatible with Python 3

Bug: https://crbug.com/gerrit/12145
Change-Id: I01d1ef96ff7b474f55ed42ecc13bd5943006d3b5
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/251833


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Rostislav Krasny <rostigm@gmail.com>