Pyroma installation is slow on Py3, so just do it for Py2

Pyroma installation is a lot slower for Python 3. For example:
```
pypy    8s
pypy3   70s
2.6     3s
2.7     2s
3.2     111s
3.3     82s
3.4     86s
```
Let's skip the installation on Python 3. The test will also skip.

Reported to Pyroma: https://bitbucket.org/regebro/pyroma/issue/22/installation-slow-for-python-3