fix#561
* Add support for Literal annotation
* Updated requirements.txt
* incorporating feedback
* skip typing_extensions tests if not installed
* missed a spot
* address feedback
* Make work with python 3.6
* Work for *both* 3.6 and 3.7
* incorporate feedback
* fixed naming and quotes
* Trying to fix LGTM bot issue
* Added generic functionality
* Skip tests in python 3.6
* double quote -> single quote
* Simplified, with more comprehensive tests
* double quote -> single quote
* Remove unintentional file
* Add caching
* don't cythonize generics.py
* Make work with mypy
* Remove __parameters__
* double quote -> single quote i'll remember one day
* More cleanup and validation
* Removed unwanted file
* A little more cleanup, and finish the PR
* Add proper inheritance
* Added note about inheritance to docs
* Added error for double-parameterizing
* Should build for python3.7
* Works with both 3.6 and 3.7
* Fixed bug with caching for single argument
* handle __name__ for generic models
* double quote -> single quote
* Updated error messages
* working on parsers
* starting parse tests
* proper tests for parse
* adding parse docs
* tweaks and history
* add test for datetime direct, fix tests
* tweak docs