Prior to this commit, importing a module that exists but has a syntax error
or some other problem that happens at import time would result in a
potentially-incomplete module object getting added to sys.modules.
Subsequent imports would use that object, resulting in confusing error
messages that hide the root cause of the problem.
This commit fixes that issue by removing the failed module from sys.modules
using the new NLR callback mechanism.
Note that it is still important to add the module to sys.modules while the
import is happening so that we can support circular imports just like
CPython does.
Fixes issue #967.
Signed-off-by: David Grayson <davidegrayson@gmail.com>
David Grayson