Emit missing-required-key errors in schema-declaration order (#484)

[uttam12331]

Summary

Closes #484.

Required keys were collected into a set, so the MultipleInvalid raised for missing required keys came out in an arbitrary, hash-seed-dependent order. The reporter's two-key test failed ~1 in 3 runs:

S = Schema({Required('value1'): str, Required('value2'): str, Required('value3'): str})
S({})  # errors sometimes [value1, value2, value3], sometimes [value2, value1, value3], ...

Change

Store the required keys in an insertion-ordered dict (used as an ordered set) instead of a set, and replace the set.discard(...) calls with dict.pop(..., None). The leftover-required-key errors are now emitted in the order the keys were declared in the schema. No other behavior changes.

Verified across several PYTHONHASHSEED values — before, the error order varied per seed; after, it is always declaration order.

Tests

Added test_required_key_error_order_is_stable (5 required keys, asserts the error paths equal the declaration order — an accidental correct ordering on the old code would be ~1 in 120). Full suite passes (170 passed); black --check and flake8 clean. Added a CHANGELOG entry.

[alecthomas]

This has conflicts that will need to be resolved.

[uttam12331]

Thanks @alecthomas — rebased on latest master and resolved the CHANGELOG conflict (moved the #484 entry under the new [Unreleased] section). Full suite still passes (182 passed). Ready for merge.