python: Fix inequality comparisons

python: Fix inequality comparisons

On Python 3, executing `foo != bar` will first try to call
foo.__ne__(bar), and fallback on the opposite result of foo.__eq__(bar).

Python 2 does not do that.

As a result, those __eq__ methods were never called, when we were
testing for inequality.

Expliclty adding the __ne__ methods fixes this issue, in a way that is
compatible with both Python 2 and 3.

However, this means the __eq__ methods are now called when testing for
`foo != None`, so they need to be guarded correctly.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>