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/tests/__init__.py

@@ -0,0 +1 @@
+"""Tests for the Python variant."""