tox.ini: Add "pep8" target

```
❯ tox -e pep8
GLOB sdist-make: /Users/marca/dev/git-repos/readme/setup.py
pep8 inst-nodeps: /Users/marca/dev/git-repos/readme/.tox/dist/readme-0.3.0.zip
pep8 runtests: PYTHONHASHSEED='3029599132'
pep8 runtests: commands[0] | flake8 .
___________________________________________________________________________________ summary ____________________________________________________________________________________
  pep8: commands succeeded
  congratulations :)
```