The proposed PR adds missed checks for errors and PyErr_Clear() calls in PyImport_Cleanup().

It may be worth to use PyErr_WriteUnraisable() in place of PyErr_Clear() (newly added and existing) in most cases. But this can add an unexpected noise, and I don't wont to do this at this stage of 3.7. It could be done in 3.8 though, and backported to 3.7 later.

I'm not sure that PyImport_Cleanup() works properly if sys.modules is not a dict. At this stage a custom mapping will likely be broken when most of modules are destroyed.