Fix black exclude. (#1606)
Fix black exclude. (#1606)

The prior setup was skipping all files:
```
$ tox -qq -eformat-run
No Python files are present to be formatted. Nothing to do 😴
Skipped 8 files
```

Now we get them all looked at:
```
$ tox -qq -eformat-run
All done! ✨ 🍰 ✨
182 files left unchanged.
Skipped 8 files
```
jsirois/pex,jsirois/pex,pantsbuild/pex,pantsbuild/pex