As the project is Python 3 only, can remove the compatibility shims in
compat.py.

Type checking has been simplified where it can:
  - str is iterable
  - bytes is iterable
  - use isinstance instead of issubclass

The remaining function bytes_from_int() has been moved to utils.py.

This assumes all direct calls to private methods (leading underscore in name)
are from pyjwt code, hence stacklevel=3 nor 2.

pyupgrade is a tool to automatically upgrade Python syntax for newer
versions of the language. Running pyupgrade removes several
Python-2-isms that are no longer necessary now that the project is
Python 3 only.

https://github.com/asottile/pyupgrade

* Setup pre-commit hooks

* Run initial `tox -e lint`

* Fix package name

* Fix .travis.yml

Per recent upstream fix to typeshed, json.dumps() cls argument should be
optional type JSONEncoder.

https://github.com/python/typeshed/commit/8e0d288ea49a34f9bd21b1598ec487414a339a1f

Fixes mypy error:

    jwt/api_jws.py:102: error: Argument "cls" to "dumps" has incompatible type "Optional[Callable[..., Any]]"; expected "Optional[Type[JSONEncoder]]"
    jwt/api_jwt.py:61: error: Argument "cls" to "dumps" has incompatible type "Optional[Callable[..., Any]]"; expected "Optional[Type[JSONEncoder]]"

* Move ABCs imports to compat.py to reuse the imports from other modules

* Import collection ABC's from correct module

* Fix #351 by reverting argument name changes for .decode()

* Update CHANGELOG and bump version to 1.6.4

* Add mypy to Travis and a simple type hint to _get_default_options

* Make flake8 accept unused import required by mypy

* Add typing to encode and decode, create encode_bytes in JWS to differentiate from encode in JWT

* Use Union type to describe both types of payload