Make autoreload extension work on Python 3.

[takluyver]

Some changes for Python 3 in general, and some changes for the .pyc repository directories introduced in Python 3.2 (PEP 3147)

Closes gh-846

[takluyver]

I think this check is now superfluous, but I'd like someone else to look at it before I remove it. source_from_cache raises ValueError unless the extension is .pyc or .pyo.

[fperez]

Removing it looks clean to me.

[fperez]

Looks good, go ahead and merge after removing that try/except if you want. Thanks!

[takluyver]

Sorry, I meant the if ext != PY_COMPILED_EXT check, not the try/except. The try/except is essential, as I spent a while working out.

[fperez]

Ah, OK. Actually, I imagine if you are running from a read-only filesystem with only sources (.py) installed, ext could be .py, no? That would justify keeping that if ext != PY_COMPILED_EXT check, it seems to me.

[takluyver]

if ext is .py, it gets set to PY_COMPILED_EXT a few lines above.

The only way I think it could now be called is if we somehow end up with a .pyo filename in a normal process, or a .pyc in an optimised one. And if that did happen, I don't think it would need to be skipped, anyway.

[fperez]

OK, then go ahead with the removal and merge. Thanks!