cext: correct the argument handling of `b85encode()` The type stub indicated that this argument is `Optional`, which implies None is allowed. I don't see in the documentation where that's the case for `i`[1], and trying it in `hg debugshell` resulted in the method failing with a TypeError. I guess it was typed as an `int` argument because the `p` format unit wasn't added until Python 3.3[2]. In any event, 2 clients in core (`pvec` and `obsolete`) call this with no argument supplied, and `mdiff` calls it with True. So I guess we've avoided the None arg case, and when no arg is supplied, it defaults to the 0 initialization of the `pad` variable in C. Since the `p` format unit accepts both `int` and None, as well as `bool`, I'm not bothering to bump the module version- this code is more permissive than it was, in addition to being more correct. Interestingly, when I first imported the `cext` and `pure` methods in the same manner as the previous commit, it dropped the `Optional` part of the argument type when generating `util.pyi`. No idea why. [1] https://docs.python.org/3/c-api/arg.html#numbers [2] https://docs.python.org/3/c-api/arg.html#other-objects