typing: add type hints to the platform `cachestat` classes

util: fix the signature of observedbufferedinputpipe._fillbuffer() Flagged by PyCharm, since it didn't match the signature of the method being overridden. The default value in the superclass is also `_chunksize`, and I suspect that the amount read from `osread` should be limited to what is passed in. Only one caller (`bufferedinputpipe.unbufferedread()`) passes this argument, and it passes the max of `_chunksize` and whatever it was passed.

tests: drop some obsolete py2 handling in util.py doctest Flagged by PyCharm while inspecting imports from the platform modules.

typing: add type hints to the common posix/windows platform functions These are done in sync because some platforms have empty implementations, and it isn't obvious what the types should be without examining the other. We want the types aligned, so @overload definitions that differ aren't generated. The only differences here are the few methods that unconditionally raise an error are marked as `NoReturn`, which doesn't seem to bother pytype. A couple of the posix module functions needed to be updated with a modern ternary operator, because pytype seems to want to use the type of the second object in the old `return x and y` style.

typing: add type hints to the posix platform module matching win32.py

typing: add type hints to mercurial/win32.py These are the low level functions that are imported by the mercurial.windows module, which is in turn imported by mercurial.utils as the platform module. Pretty straightforward, but pytype inferred very little of it, likely because of the heavy ctypes usage. It also seems to trigger a pytype bug in procutil, now that it has an idea of the underlying function type, so disable that warning to maintain a working test.

windows: drop some py2 registry module importing The comment was actually backwards- `winreg` is importable on py3, and is already imported by mercurial/windows.py.

typing: add type hints to the platform specific scm modules Surprisingly, pytype struggled to figure out the return types in the posix functions.