Port codebase from Python 2 to Python 3

Replace Python 2-only constructs throughout libs/: print statements,
ConfigParser -> configparser, sets.Set -> builtin set, string.split ->
str.split, xrange -> range, dict.has_key()/.iteritems() -> in/.items(),
old-style raise Exception, "msg" syntax, and __cmp__ -> __eq__/__lt__
via functools.total_ordering (Python 3 dropped cmp()/__cmp__, which
max()/.sort() rely on). Also cleaned up mixed tab/space indentation
that Python 3's stricter tokenizer rejects.

Fixed two latent bugs that only surfaced once end-to-end runs were
possible under Python 3's stricter error handling:
- HandleBestellung() compared G.Anzahl (a bound method) to an int
  instead of calling G.Anzahl() - silently "worked" under Python 2's
  permissive cross-type ordering, raises TypeError under Python 3.
- LeuteAllein() raised KeyError for VIP-group members, who are never
  added to the regular Gruppen container; now skips people with no
  group entry instead of crashing.

Verified end-to-end: bin/platz.sh --indir work/test1 and --indir
work/test2 (VIP group included) both run to completion and produce
correctly formatted output.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -5,10 +5,9 @@ given group memberships, table sizes/neighbor relationships, and a configurable
 scheme. It reads a guest list (XML) and a table layout (INI), searches for a good seating
 using a small custom GA framework, and writes the result to text/CSV output files.
 
-**This is Python 2 code** (print statements, `ConfigParser`, `sets.Set`, `UserList`,
-`string.split`, old-style `raise Exception, "msg"`, `xrange`, `dict.has_key`,
-`dict.iteritems`). There are no third-party dependencies (standard library only), so
-`requirements.txt` is empty — it exists only so `bin/install_py` has something to install.
+**This is Python 3 code** (migrated from an original Python 2 implementation). There are no
+third-party dependencies (standard library only), so `requirements.txt` is empty — it
+exists only so `bin/install_py` has something to install.
 
 ## Project structure