Pydantic

* 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

* fix tests for Python 3.8, [396](https://github.com/pydantic/pydantic/pull/396) by samuelcolvin
* Adds fields to the `dir` method for autocompletion in interactive sessions, [398](https://github.com/pydantic/pydantic/pull/398) by dgasmith
* support `ForwardRef` (and therefore `from __future__ import annotations`) with dataclasses, [397](https://github.com/pydantic/pydantic/pull/397) by samuelcolvin

* **breaking change** (maybe): more sophisticated argument parsing for validators, any subset of
`values`, `config` and `field` is now permitted, eg. `(cls, value, field)`,
however the variadic key word argument ("`**kwargs`") **must** be called `kwargs`, [388](https://github.com/pydantic/pydantic/pull/388) by samuelcolvin
* **breaking change**: Adds `skip_defaults` argument to `BaseModel.dict()` to allow skipping of fields that
were not explicitly set, signature of `Model.construct()` changed, [389](https://github.com/pydantic/pydantic/pull/389) by dgasmith
* add `py.typed` marker file for PEP-561 support, [391](https://github.com/pydantic/pydantic/pull/391) by je-l
* Fix `extra` behaviour for multiple inheritance/mix-ins, [394](https://github.com/pydantic/pydantic/pull/394) by YaraslauZhylko