Define and enforce a valid-label criterion for game objects (#944)

[d-kad]

Issues closed by this PR

• Closes [ENH]: Define and enforce criteria for legal labels in game representation #944

Description of the changes in this PR

This PR defines and enforces a single criterion for valid game-object labels, centrally at the C++ level.

A label is valid if it consists only of printable ASCII characters and spaces, does not begin or end with a space, and does not contain two consecutive spaces. The empty label remains valid. The definition is enforced in one place (IsValidLabel/CheckLabel in game.h) so that widening it to permit Unicode in 17.0 (#862) is a localised change.

• CheckLabel is called from all six SetLabel methods (outcome, action, infoset, strategy, player, node) and from the GameStrategyRep constructor.
• In pygambit, the six SetLabel declarations and NewStrategy now map the C++ exception to ValueError (except +ValueError); an invalid label raises ValueError.
• add_strategy now encodes the label as ASCII, consistent with the other accessors; a non-ASCII label fails at the boundary with UnicodeEncodeError.

Tests add per-type valid/invalid/non-ASCII coverage (constants in tests/games.py).

[tturocy]

@d-kad This looks good for a start. I did check and there are some paths in the GUI which aren't catching exceptions, so if we merge this without adding at least some crude defensive handlers we're creating a bad regression (it completely crashes the GUI!) so I need to look at those first before we can accept (unfortunately we can't treat them entirely as separate issues as #945 wishes were true!)