Bump version to 3.2-alpha
IMPORTANT: This means that the master branch is now considered feature-complete
for the upcoming 3.2 release, and thus in *feature freeze*.
Unless explicitly allowed by project maintainers, no new feature PRs will be
considered for merge until Godot 3.2-stable is released. Current PRs made
before the feature freeze will still be reviewed and potentially merged before
the beta stage, if deemed satisfactory.
Rémi Verschelde