Conditionally import weakreset for Python versions < 2.7

I've modified __init__.py to conditionally import wekrefst. Pleas have a look because I've never done this before.
This should use what is confusingly called an "absolute import" in Python:

```
from .utils.compat import weakref
```

(Note the period in front).  Without the period, Python will search in all directories on the Python path for a module or package called "utils".  If the user ever installed a package by that name, they would get that instead of the `utils` package in `sphere`.  The leading period says "look only in the same directory as this module".