py3: minor fixes for sage.rings.number_field

[embray]

These are the remaining fixes to this package in my python3 branch that haven't been upstreamed yet.

Apparently I already had a hash implementation for the Order class, but this supplements the docstring with additional tests I had written.

Regarding the timestr() function, I would normally be wary of removing any non-underscored function without a deprecation, but in this case I believe it is justified:

1. It was only ever used as a utility function in a different module that was removed 8 years ago (#9359), at which point it became dead code. It should have been removed with the rest of #9359.

2. Nothing else uses or has ever used it.

3. It doesn't have anything to do with the rest of the surrounding code; there's no reason anyone would look to that module for such a function.

4. It is not well implemented even for what it's supposed to do. In the remote chance that someone is actually using this in their code they almost certainly have bigger problems ;)

[chapoton]

ok, let it be

[embray]

Ack, I missed a range(...) that needed updating, but was not covered by the tests in that module. I'll just include it with my next update...