Accept Mapping inputs for dict schemas

[deepakganesh78]

Fixes #299

Reproduction

A dictionary schema currently rejects collections.abc.Mapping implementations that are not dict subclasses:

from collections.abc import Mapping
from voluptuous import Schema

class QueryArgs(Mapping):
    def __init__(self, data):
        self._data = data
    def __getitem__(self, key):
        return self._data[key]
    def __iter__(self):
        return iter(self._data)
    def __len__(self):
        return len(self._data)

Schema({"page": int})(QueryArgs({"page": 1}))

On current master this raises MultipleInvalid: expected a dictionary.

Root cause

Schema._compile_dict() only accepted dict instances even though the schema type already treats collections.abc.Mapping as a valid dictionary-like schema and mappings expose the .items() / membership APIs used by validation.

Fix

Accept collections.abc.Mapping inputs in dictionary schemas. Existing dict and dict subclass output behavior is preserved; generic Mapping inputs produce a plain dict, avoiding assumptions that arbitrary mapping classes can be constructed empty and mutated.

Compatibility notes

The existing error message for non-mapping inputs remains expected a dictionary. This only broadens accepted inputs for objects implementing the standard Mapping protocol.

Validation

• Minimal reproducer failed before the fix with MultipleInvalid: expected a dictionary and succeeds after the fix.
• Regression check with fix reverted: python -m pytest voluptuous\tests\tests.py::test_schema_accepts_mapping -q fails with MultipleInvalid: expected a dictionary.
• Targeted regression with fix: python -m pytest voluptuous\tests\tests.py::test_schema_accepts_mapping -q -> 1 passed.
• Full suite: python -m pytest -> 183 passed.
• Formatting/lint/type checks: black --check voluptuous\schema_builder.py voluptuous\tests\tests.py -> unchanged; isort --check voluptuous\schema_builder.py voluptuous\tests\tests.py -> passed; flake8 --doctests voluptuous\schema_builder.py voluptuous\tests\tests.py -> passed; mypy voluptuous -> success, no issues found in 10 source files (mypy 2.3.0 also notes the repo's configured python_version = 3.9 is below its supported range on Python 3.13).

[alecthomas]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 7cd2ce0e2e

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