scmposix: don't subscript IOError This warning disabling has been in place since late 2019 in 667f56d73ceb. We should have had some py3 support at the time, but both pytype complains and subscripting a real FileNotFoundError generated in `hg debugshell` crashed, so maybe this fixes a problem. It looks like all other instances of subscripting exceptions have been replaced (at least as far as greping for `== errno.` revealed).

typing: add type hints to pycompat.bytestr The problem with leaving pytype to its own devices here was that for functions that returned a bytestr, pytype inferred `Union[bytes, int]`. It now accepts that it can be treated as plain bytes. I wasn't able to figure out the arg type for `__getitem__`- `SupportsIndex` (which PyCharm indicated is how the superclass function is typed) got flagged: File "/mnt/c/Users/Matt/hg/mercurial/pycompat.py", line 236, in __getitem__: unsupported operand type(s) for item retrieval: bytestr and SupportsIndex [unsupported-operands] Function __getitem__ on bytestr expects int But some caller got flagged when I marked it as `int`. There's some minor spillover problems elsewhere- pytype doesn't seem to recognize that `bytes.startswith()` can optionally take a 3rd and 4th arg, so those few places have the warning disabled. It also flags where the tar API is being abused, but that would be a tricky refactor (and would require typing extensions until py3.7 is dropped), so disable those too.

pycompat: explicitly prefix builtin attr usage with `builtins.` It doesn't seem like this would fix any bug, because the wrapped functions that take bytes instead of str are defined after these calls. But PyCharm was flagging the second and third uses, saying "Type 'str' doesn't have expected attribute 'decode'". It wasn't flagging the first, but I changed it for consistency.

typing: add type hints to global variables in mercurial/pycompat.py The way `osaltsep` and `sysexecutable` were defined, pytype determined them to be `Union[bytes, str]`. This was a problem because that cascaded to all of the callers, and also because it couldn't be annotated as bytes on the initial assignment. Therefore, we use a ternary operator. The documentation says that `sys.executable` can either be None or an empty string if the value couldn't be determined. We opt for an empty string here because there are places that blindly pass it to `os.path.xxx()` functions, which crash if given None. Other places test `if pycompat.sysexecutable`, so empty string works for both.

windows: drop an unused method The only caller was removed in 563eb25e079b.

typing: add type hints to the prompt methods in mercurial/ui.py The @overloads allow for the callers that pass a non-None `default` to not have to worry about handling a None return to appease pytype.

ui: split the `default` arg out of **kwargs for the internal prompt method This arg was required anyway, based on how it was accessed. Having it separate allows it to be typed though, and this will simplify things for the callers- if a non-None `default` is passed, the return can never be None. That can be expressed with `@overload` when the arg can be typed, but that's not possible when it is rolled up in **kwargs. The default value is simply copied from the public `prompt()` above it.