Release Procedure

Create a branch release-X.Y.Z from the latest master.
Manually update the CHANGELOG.rst and commit.
Open a PR for this branch targeting master.
After all tests pass and the PR has been approved by at least another maintainer, publish to PyPI by creating and pushing a tag:
```
git tag X.Y.Z
git push git@github.com:pytest-dev/py X.Y.Z
```
Wait for the deploy to complete, then make sure it is available on PyPI.
Merge your PR to master.