Dataslots

**Features**
* Added official support for python 3.10 and pypy 3.7-3.9.
* Added official support for version 5 of pickle protocol.
* Added support for data descriptors.
* Added better typing for mypy support.

**Bugfixes**
* Raise `TypeError` is `__slots__` are already defined.
* Raise `TypeError` if class is not `dataclass`

**Removals**
* Drop support for python 3.6.

* `with_slots` is renamed to `dataslots` (`with_slots` is marked as deprecated and will be erased in future releases)
* fixed creating class with custom metaclass defined
* added workaround for pickling dataclasses with slots (https://bugs.python.org/issue36424)
* added official support for python 3.8, 3.9 and pypy3.6

Preview of version 1.0.2 deployed for the first time using Travis.

* Support for python 3.6 and 3.8-dev
* More unittests

Initial version of dataslots
* __slots__ working with dataclass decorator
* support for __dict__ and __weakref__
* support for inheritance
* working on python 3.7.0b2 (be careful with bpo-33100)