Add the Python variant, agreeing with Go byte for byte

A second, independent implementation of the same contract, in the same repo:

- swehockey/dom.py builds the tree on html.parser, so the variant has no
  runtime dependencies; the only HTML5 behaviour it depends on - stray <tr>
  landing in an implicit <tbody> - is handled explicitly by table_rows().
- parse.py mirrors the Go layout profiles, dual-render checks, carry-forward and
  invariants; a DriftError is raised in the same places with the same advice.
- cli.py offers the same three commands, flags and exit codes as the Go main.
- tests/ assert against the Go golden files, so the suite is also a
  cross-implementation check. 23 offline tests plus an opt-in live test.

Verified against the live site: JSON and CSV are byte-identical to the Go
output (364 games, 102536 bytes / 365 lines), and the full archive crawl gives
the same 27 seasons, 2119 games and 1779 played.

Also split the DriftError hint out in Go, so a golden mismatch no longer tells
the reader to extend layouts[].

python/pyproject.toml

@@ -0,0 +1,30 @@
+[build-system]
+requires = ["setuptools>=68"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "swehockey-schedule"
+version = "1.0.0"
+description = "Deterministic crawler for the stats.swehockey.se schedule table"
+readme = "README.md"
+requires-python = ">=3.10"
+license = { text = "MIT" }
+authors = [{ name = "Mats Tornberg" }]
+keywords = ["hockey", "swehockey", "shl", "crawler", "scraper", "deterministic"]
+classifiers = [
+  "Programming Language :: Python :: 3",
+  "Topic :: Text Processing :: Markup :: HTML",
+]
+
+# No runtime dependencies on purpose: the HTML tree is built on html.parser
+# from the standard library.
+dependencies = []
+
+[project.scripts]
+swehockey-schedule = "swehockey.cli:main"
+
+[project.urls]
+Homepage = "https://git.k6n.net/mats/swehockey"
+
+[tool.setuptools]
+packages = ["swehockey"]