Derive the version from git with verri

Use verri's strawberry flavour through hatchling's code version source, so
versions look like 2026.10.21.0: the HEAD commit date plus the number of
earlier commits that day.

Container images build the sdist in a separate stage that has the git
repository, so they get the same version instead of a +vcs.missing one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -2,7 +2,7 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 [build-system]
-requires = ["hatchling>=1.27", "hatch-fancy-pypi-readme>=24.1"]
+requires = ["hatchling>=1.27", "hatch-fancy-pypi-readme>=24.1", "verri>=2026.10.11"]
 build-backend = "hatchling.build"
 
 [project]
@@ -31,11 +31,15 @@ Source = "https://git.bartbroe.re/bartbroere/peo"
 [project.scripts]
 peo = "peo.__main__:run"
 
+# The version is derived from the date of the HEAD commit, plus the number of
+# commits made earlier that day, like 2026.10.21.0. See version.py.
 [tool.hatch.version]
-path = "peo/__init__.py"
+source = "code"
+path = "version.py"
+expression = "strawberry()"
 
 [tool.hatch.build.targets.sdist]
-include = ["/peo", "/README.md", "/LICENSE"]
+include = ["/peo", "/README.md", "/LICENSE", "/version.py"]
 
 [tool.hatch.build.targets.wheel]
 packages = ["peo"]