Treat global dunder names as always used

[anntzer]

When using a package versioning helper such as setuptools_scm, it is quite natural to write, in a package's toplevel __init__.py, something like

from ._version import version as __version__

(assuming one uses setuptools_scm's write_to option rather than pkg_resources, as the latter can be quite slow).

Currently, pyflakes complains that __version__ is an unused name. Obviously, one can placate pyflakes by writing (over two lines) from . import _version; __version__ = _version.version, but that seems a bit silly: I would instead suggest that pyflakes consider all global-level dunders as automatically used (in the sense "they have a special or metadata role anyways"), even if they appear not so.

[asmeurer]

pyflakes currently requires using __all__ in __init__.py to avoid unused name errors (I'm not sure, but I think it's a good idea for even __version__ to go in __all__).

[anntzer]

Actually, I think putting __version__ in __all__ is a pretty bad idea: someone who does from foo import * (say, in the bar) module will now also set bar.__version__ (as it's a string, it doesn't keep track of its origin module, so third party tools will use that as the bar module version.
(Yes, import * is a bad idea but we're talking about __all__ anyways...)