REL: do a better job of including files in repo root in sdist

Not only environment.yml is added, setup.cfg was missing too. Current
result of `ls -a` in the top-level directory of the unpacked sdist:

```
.                     LICENSE.txt              pytest.ini                site.cfg.example
..                    linter_requirements.txt  README.md                 test_requirements.txt
benchmarks            MANIFEST.in              release_requirements.txt  THANKS.txt
doc                   numpy                    runtests.py               tools
doc_requirements.txt  pavement.py              setup.cfg                 tox.ini
environment.yml       PKG-INFO                 setup.py                  versioneer.py
INSTALL.rst.txt       pyproject.toml
```

0 files changed, 0 insertions, 0 deletions