project: fix bytes/str encoding when updating git submodules

Since tempfile.mkstemp() returns a file handle in binary mode,
make sure we turn our strings into bytes before writing.

Bug: https://crbug.com/gerrit/12043
Change-Id: I3e84d595e84b8bc12a1fbc7fd0bb3ea0ba2832b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254393
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -2073,7 +2073,7 @@ class Project(object):
       gitmodules_lines = []
       fd, temp_gitmodules_path = tempfile.mkstemp()
       try:
-        os.write(fd, p.stdout)
+        os.write(fd, p.stdout.encode('utf-8'))
         os.close(fd)
         cmd = ['config', '--file', temp_gitmodules_path, '--list']
         p = GitCommand(None, cmd, capture_stdout=True, capture_stderr=True,