dropped `!=`, since `==` should be sufficient (as discussed)

I've extended the ordering test a bit, and I'm seeing that the ordering of the versions depends on the order in which the version expressions are added.

E.g., when adding the following (in this order): `> 3.0.0`, `== 1.0.0`, `>= 2.5.0`, `> 2.0.0`, then the ordered set of version expressions is:

```python
['> 2.0.0', '>= 2.5.0', '== 1.0.0', '> 3.0.0']
```

But, when they're adding in the following order: `== 1.0.0`, `> 2.0.0`, `> 3.0.0`, `>= 2.5.0`

the result is:

```python
['>= 2.5.0', '> 3.0.0', '> 2.0.0', '== 1.0.0']
```

which is a totally different order, even though the set of version expressions is the same (and seems pretty reasonable).
This seems to be indicating there's a bug in the ordering implementation, because it shouldn't matter in which way section markers are added, right? Or does it matter?
Do the examples above make sense to you, and if so, please explain. ;-)