cmdutil: switch the `mode` on `cmdutil.makefileobj()` to str I think the typing around whether `open()` returns `IO[bytes]` or `IO[str]` hinges on the content of the mode string. Converting from bytes instead of using a literal can suppress that (though PyCharm currently complains about this). Instead, we can mandate the use of a (vastly reduced) set of mode options. For now, none of the 3 callers provide this argument, so it's not a big deal. Ideally, this would always enforce binary mode. There's a little extra typing required to pull this off. The `_unclosablefile` class can't subclass `typing.BinaryIO`, because there were a bunch of test failures around writing to stdout. Strangely, pytype didn't complain that the abstract methods on `typing.BinaryIO` weren't overridden in this case. Whatever was going on, it's a simple proxy class, so we can just cast to the expected type in the one place it is used.