Avoid using built-in vars() which fails on objects without dict

[Tronic]

Better handling of objects using __slots__.

Fix #6

[ahopkins]

Looks simple and straight-forward enough. Two points--and they are larger project-wide items for consideration:

1. No testing.
2. I would consider making it _safe_vars since it is not an API intended for public consumption.

[Tronic]

@ahopkins Not likely I'll have time to implement tests. Public APIs are imported in __init__.py and the submodules don't use that underscore convention for their helper functions (such as prettyvalue in that same module).

[ahopkins]

@ahopkins Not likely I'll have time to implement tests. Public APIs are imported in __init__.py and the submodules don't use that underscore convention for their helper functions (such as prettyvalue in that same module).

I didn't think you would get tests started for this PR, but something we should consider in future.