PEP-8 compliance

## Before

```
(pytest-sugar)
marca@marca-mac2:~/dev/git-repos/pytest-sugar (master●)
$ pep8 --quiet --count --statistics pytest_sugar.py
pytest_sugar.py
1       E221 multiple spaces before operator
3       E225 missing whitespace around operator
1       E228 missing whitespace around modulo operator
4       E231 missing whitespace after ':'
5       E302 expected 2 blank lines, found 1
3       E303 too many blank lines (2)
1       E401 multiple imports on one line
5       E501 line too long (84 > 79 characters)
5       W291 trailing whitespace
28
```

## After

```
(pytest-sugar)
marca@marca-mac2:~/dev/git-repos/pytest-sugar (pep8●)
$ pep8 --quiet --count --statistics pytest_sugar.py
```