vfs: use @abstractmethod instead of homebrewing abstract methods The latter confuses PyCharm after adding more type annotations when, for example, `abstractvfs.rename()` calls `_auditpath()`- the latter unconditionally raised an error, so PyCharm thought the code that came after is unreachable. It also tricked pytype into marking the return type as `Never`, which isn't available until Python 3.11 (outside of `typing_extensions`). This also avoid PyCharm warnings that the call to the superclass constructor was missed (it couldn't be called because it raised an error to prevent instantiation). The statichttprepo module needed to be given an override for one of the abstract methods, so that it can be instantiated. In `abstractvfs`, this method is only called by `rename()`, so I think we can leave this empty. We raise an error in case somebody accidentally calls it in the future- it would have raised this same error prior to this change. I couldn't wrangle `import-checker.py` into accepting importing `ABC` and `abstractmethod`- for each subsequent import, it reports something like: stdlib import "contextlib" follows local import: abc I suspect the problem is that near the `if fullname != '__future__'` check, if the module doesn't fall into the error case, `seenlocal` gets set to the module name. That causes it to be treated like a local module on the next iteration, even though it is in `stdlib_modules`.