Tox issues with Python 3.6 and 3.7

[Hamish McIntyre-Bhatty <hamishmb@live.co.uk>]

Hi there,

This is a follow-up to Marco Atzeri and I talking about tox in the 
Python 3.9 email thread. I have just started trying to use tox for 
testing python-imaging.

I have found that tox has some issues on Python 3.6 and 3.7, namely that:

3.6:

typing-extensions (on PyPI) is required for tox to run (I installed with 
pip3.6). This is not available in the Cygwin repositories.

Then, platformdirs is needed. This is available in the Cygwin 
repositories but not for 3.6.

I then decided not to bother supporting 3.6 seeing as it is EOL now anyway.

3.7:

As above typing-extensions is required, which I installed with pip3.7.

Tox then says it can't find the "interpreter for Builtin discover of 
python_spec='python3.7m'"

It also seems to be trying to build Pillow again inside a virtualenv, 
but doesn't find jpeg headers/libraries, which are installed.

Marco, I assume you didn't have these problems when you tested, or you 
found a way to fix them. Having never used tox before, I'm at a bit of a 
loss, any ideas?

Hamish