This code (reduced from an example I had of trying to debug code including the module-level code of already imported modules), raises a SystemError:
import builtins
from importlib.machinery import PathFinder
from importlib.util import find_spec
import importlib
import sys
import _imp
dct={}
def copy_module(module):
new = type(sys)(module.__name__)
new.__dict__.update(module.__dict__)
return new

dct["__builtins__"] = b = copy_module(builtins)

spec = PathFinder.find_spec("_bootstrap",importlib.__path__)
source_bootstrap = type(sys)("_bootstrap");
spec.loader.exec_module(source_bootstrap);
source_bootstrap.__name__ = "importlib._bootstrap";    
new_sys = copy_module(sys)

new_sys.path_hooks = []
new_sys.meta_path = []
new_sys.modules = {
    "importlib._bootstrap":source_bootstrap,
    "importlib._bootstrap_external":importlib._bootstrap_external,
}
b.__import__ = source_bootstrap.__import__
source_bootstrap._install(new_sys,_imp)
dct["__file__"]=__file__
exec("open(__file__)",dct)

The actual file passed to the open function doesn't matter, as long as it would work

I couldn't reproduce this on 3.7, but I can confirm that 3.6 gives a SystemError with the above code:

$ ./python _issue30626.py
Traceback (most recent call last):
  File "_issue30626.py", line 30, in <module>
    exec("open(__file__)",dct)
  File "<string>", line 1, in <module>
SystemError: <class '_io.TextIOWrapper'> returned NULL without setting an error

It's plausible this was fixed as part of the interpreter startup refactoring, so I doubt we're going to able to easily isolate the specific change that fixed it :(

That said: given that we know it *is* fixed somewhere in 3.7, it would likely be useful to check the assumption that the startup refactoring fixed it by going to the last commit before that landed and seeing if the error still occurs.

In rare circumstances PyImport_Import() could return NULL without raising an error.

It *isn't* fixed in 3.7.

New changeset 145541c by Serhiy Storchaka in branch 'master':
bpo-30626: Fix error handling in PyImport_Import(). (bpo-2103)
145541c

New changeset fab05de by Serhiy Storchaka in branch '3.6':
[3.6] bpo-30626: Fix error handling in PyImport_Import(). (GH-2103) (bpo-2221)
fab05de

New changeset 263dcc3 by Serhiy Storchaka in branch '3.5':
[3.5] bpo-30626: Fix error handling in PyImport_Import(). (GH-2103) (bpo-2222)
263dcc3