PEP 593 has been accepted some time ago. I got a green light for merging
this from Till, so I went ahead and combined the code contributed to
typing_extensions[1] and the documentation from the PEP 593 text[2].

My changes were limited to:

* removing code designed for typing_extensions to run on older Python
  versions
* removing some irrelevant parts of the PEP text when copying it over as
  documentation and otherwise changing few small bits to better serve
  the purpose
* changing the get_type_hints signature to match reality (parameter
  names)

I wasn't entirely sure how to go about crediting the authors but I used
my best judgment, let me know if something needs changing in this
regard.

[1] https://github.com/python/typing/blob/8280de241fd8c8afe727c7860254b753e383b360/typing_extensions/src_py3/typing_extensions.py
[2] https://github.com/python/peps/blob/17710b879882454d55f82c2d44596e8e9f8e4bff/pep-0593.rst