As of setuptools 46.4.0, this extracts the attribute value statically
using the ast module, if possible. This allows it to work properly even
if the attribute is stored in a file that cannot be imported at setup
time (e. g. because of dependencies that might not be installed yet).
Reading the version number using attr: rsrcfork.__version__ will no
longer work properly if rsrcfork has non-stdlib dependencies at import
time, because setuptools needs to be able to import rsrcfork and read
the version number before the dependencies are installed.
As a workaround, our setup.py now manually parses the version number
from rsrcfork/__init__.py using the ast module.
