Fix TypeError building key error message for a tuple key (#253)

[vineethsaivs]

Fixes #253.

When a key's value fails validation, the key error message is built with:

k = "Key '%s' error:" % nkey

If nkey is a tuple, % interprets it as the format arguments rather than a single value. For the empty tuple this raises TypeError: not enough arguments for format string, and for other tuple keys it would consume the tuple's elements as multiple format args.

>>> Schema({(): [(str,)]}).is_valid({(): ["foo", ("bar",)]})
TypeError: not enough arguments for format string

Wrapping the key in a single-element tuple (% (nkey,)) formats it as one value:

>>> Schema({(): [(str,)]}).is_valid({(): ["foo", ("bar",)]})
False
>>> Schema({("a", "b"): int}).validate({("a", "b"): "x"})
SchemaError: Key '('a', 'b')' error: ...

Added test_tuple_key_error_message_does_not_crash, which fails on master and passes with this change. All 118 existing tests still pass.

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Disclosure: prepared with AI assistance; reviewed by me and I can explain every line.