Ensure `setup.py` works again
Ensure `setup.py` works again

While `pip3 install .` still works, `python3 setup.py install` does not (and
ditto for Python2), because subpackages of `pygments` don't get installed. Even
running `find_packages` on its own has consistent behavior:

```
$ python3
>>> from setuptools import setup, find_packages
>>> find_packages(exclude=['tests'])
['pygments', 'pygments.filters', 'pygments.lexers', 'pygments.formatters', 'pygments.styles']
>>> find_packages(include=['pygments'])
['pygments']
```

The bug seems to have been introduced in
8d0828bbfc5b8deb7525a19e1037704f644563da.

Tested on macOS 10.14 with Homebrew Python 2.7.17 and 3.7.5.

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