Remove default value from optional arguments, since they are always passed.

In particular, the default value for `out` was a text stream (sys.stdout),
but the value passed to `out` is always a binary stream, which is confusing.
Get rid of the `err` argument, since it is always sys.stderr, and only used
for testing.

Change-Id: Ia289e9a97b968a0c802fc2f419397c1e494f713c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1986064Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>