typing: add some type hints for bundle2 capabilities Somewhere between hg 3dbc7b1ecaba and hg 8e3f6b5bf720, pytype determined the signature of `bundle20.capabilities` changed from `Dict[bytes, Tuple[bytes]]` to `Dict[bytes, Union[List[bytes], Tuple[bytes]]]`. First, I did try to simply be explicit about the previously inferred type, but it does seem to mix and match list/tuple now (e.g. in `writenewbundle()`). I tried changing the new list usage to tuple, but a couple of things complained, (and I think lists of one item are a little more clear to read anyway). So then I typed the dict value as `Sequence[bytes]`, which worked fine. But there's also a module level `capabilities` field, and when that's typed, pytype complains about `Sequence[bytes]` lacking `__add__`[1]. So I gave up, and just assigned it the type it wanted, with an alias. If somebody feels motivated to make the type consistent, it's simple enough to change the alias. The mutable default value to the constructor was removed to appease PyCharm's type checking on the field. (I didn't bother running the code through pytype prior to changing it, because we've previously made an effort to remove this pattern anyway.) I'm not sure why `getrepocaps()` has a default value for `role` that apparently raises an exception. It's just flagged for now so this series can land without risking additional problems. [1] https://foss.heptapod.net/mercurial/mercurial-devel/-/jobs/2466903