Data-spec-validator

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Changes:

- [fix] COND_EXIST now works with other checks correctly.
- [feature] Add new check `FOREACH`, used for any iterable.
- [behavior-change] `LIST_OF` enforce LIST type validation as well
- [behavior-change] Use `warning.warn` instead of `print`
- [internal] More type hint
- Postpone {KEY_COEXISTS, ANY_KEY_EXISTS} deprecation, will remove them in 1.9.0
- Update README

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Changes:

- Add new check COND_EXIST to support conditional key existence. More sample usage in test.
- Add deprecating messages for Check: KEY_COEXISTS, ANY_KEY_EXISTS
- Add test cases.
- DEPRECATIONS: KEY_COEXISTS, ANY_KEY_EXISTS will be removed in 1.8.0

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Changes:

- Support checks as keyword argument when building checker.
- Add test cases.

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Changes:

- Add type hints
- Support strict mode to detect unexpected key/value parameters

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Changes:

- Improve error message readability
- Provide `reset_msg_level` function to change the displayed messages
- Add `allow_none` option for `Checker` to support `None` value

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Changes:

- Change email validator regular expression (https://html.spec.whatwg.org/multipage/input.html#valid-e-mail-address)