Steps to reproduce:

$ cat a.py
import sys

class MyFinder:
    def __init__(self, path):
        if path != "__myfinder__":
            raise ImportError()

    def find_module(self, fullname, path=None):
        return None

sys.path_hooks.append(MyFinder)
sys.path.insert(0, '__myfinder__')

import time
print('OK')

$ python3.3 -V
Python 3.3.2+

$ python3.3 a.py
OK

$ python3.4 -V
Python 3.4.0rc1

$ python3.4 a.py
Traceback (most recent call last):
  File "a.py", line 14, in <module>
    import time
ImportError: spec missing loader


On Ubuntu 13.10.
python3.3 is ubuntu's package.
python3.4.0rc1 is built by myself.

Vim's custom importer does not work for it.

[vim_dev] Problem with Python 3.4
https://groups.google.com/d/msg/vim_dev/RfuOG7_Jabs/h3YzLTsRgv0J