Issue 13187: relative imports don't work when circular

This is, to say the least, annoying.

$ mkdir mypackage
$ touch mypackage/__init__.py
$ echo "from . import b" > mypackage/a.py
$ echo "from . import a" > mypackage/b.py
$ ./python -c "import mypackage.a"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "mypackage/a.py", line 1, in <module>
    from . import b
  File "mypackage/b.py", line 1, in <module>
    from . import a
ImportError: cannot import name a