Warn about `>` being an exclusive operator now.

E.g.: `pip install 'prettytable>0.7'` will no longer install
`prettytable==0.7.2`, because of PEP 440.

```
$ pip install 'prettytable>0.7'
Collecting prettytable>0.7
  The behavior of the `>` version specifier has changed in PEP 440. `>` is now an exclusive operator, meaning that >0.7 does not match >0.7.*.
  Perhaps you want `>=` instead of `>`?
  Could not find a version that satisfies the requirement prettytable>0.7 (from versions: )
  No distributions matching the version for prettytable>0.7
```