Pydantic

* Improve documentation on self-referencing models and annotations, [487](https://github.com/pydantic/pydantic/pull/487) by theenglishway
* fix `.dict()` with extra keys, [490](https://github.com/pydantic/pydantic/pull/490) by JaewonKim
* support `const` keyword in `Schema`, [434](https://github.com/pydantic/pydantic/pull/434) by Sean1708

* fix handling `ForwardRef` in sub-types, like `Union`, [464](https://github.com/pydantic/pydantic/pull/464) by tiangolo
* fix secret serialization, [465](https://github.com/pydantic/pydantic/pull/465) by atheuz
* Support custom validators for dataclasses, [454](https://github.com/pydantic/pydantic/pull/454) by primal100
* fix `parse_obj` to cope with dict-like objects, [472](https://github.com/pydantic/pydantic/pull/472) by samuelcolvin
* fix to schema generation in nested dataclass-based models, [474](https://github.com/pydantic/pydantic/pull/474) by NoAnyLove
* fix `json` for `Path`, `FilePath`, and `DirectoryPath` objects, [473](https://github.com/pydantic/pydantic/pull/473) by mikegoodspeed

* improve documentation for contributing section, [441](https://github.com/pydantic/pydantic/pull/441) by pilosus
* improve README.rst to include essential information about the package, [446](https://github.com/pydantic/pydantic/pull/446) by pilosus
* `IntEnum` support, [444](https://github.com/pydantic/pydantic/pull/444) by potykion
* fix PyObject callable value, [409](https://github.com/pydantic/pydantic/pull/409) by pilosus
* fix `black` deprecation warnings after update, [451](https://github.com/pydantic/pydantic/pull/451) by pilosus
* fix `ForwardRef` collection bug, [450](https://github.com/pydantic/pydantic/pull/450) by tigerwings
* Support specialized `ClassVars`, [455](https://github.com/pydantic/pydantic/pull/455) by tyrylu
* fix JSON serialization for `ipaddress` types, [333](https://github.com/pydantic/pydantic/pull/333) by pilosus
* add `SecretStr` and `SecretBytes` types, [452](https://github.com/pydantic/pydantic/pull/452) by atheuz

* add `IPv{4,6,Any}Network` and `IPv{4,6,Any}Interface` types from `ipaddress` stdlib, [333](https://github.com/pydantic/pydantic/pull/333) by pilosus
* add docs for `datetime` types, [386](https://github.com/pydantic/pydantic/pull/386) by pilosus
* fix to schema generation in dataclass-based models, [408](https://github.com/pydantic/pydantic/pull/408) by pilosus
* fix path in nested models, [437](https://github.com/pydantic/pydantic/pull/437) by kataev
* add `Sequence` support, [304](https://github.com/pydantic/pydantic/pull/304) by pilosus

* fix typo in `NoneIsNotAllowedError` message, [414](https://github.com/pydantic/pydantic/pull/414) by YaraslauZhylko
* add `IPvAnyAddress`, `IPv4Address` and `IPv6Address` types, [333](https://github.com/pydantic/pydantic/pull/333) by pilosus

* fix type hints of `parse_obj` and similar methods, [405](https://github.com/pydantic/pydantic/pull/405) by erosennin
* fix submodel validation, [403](https://github.com/pydantic/pydantic/pull/403) by samuelcolvin
* correct type hints for `ValidationError.json`, [406](https://github.com/pydantic/pydantic/pull/406) by layday