Add support for Python 3.8 and 3.9

[jon-betts]

This upgrades the tests and various CI files to include Python 3.8 and 3.9.

This also:

• Adds them to the declared classifiers
• Updates the contributors agreement to mention them too
• Fixes a small deprecation warning from the tests

I don't have a way of verifying the appveyor.yml or Travis changes locally, but the tests pass on those versions of Python I can install locally.

[jon-betts]

Looks like AppVeyor may not support Python 3.9. I noticed that the latest version (3.7) was missing from this file before I started.

[mmerickel]

This is great, thank you for getting into it. As far as issues with travis or appveyor it's probably fine to just disable anything that's an issue. The ultimate goal will be to move to github actions similar to some of the other projects (pyramid_tm, hupper, waitress, etc).

Is there anything I can do to kick appveyor or does 3.9 need to just be removed from it?

[jon-betts]

I'm not familiar with AppVeyor, but I've pushed a fix to comment out the latest one. They do appear to support Python 3.9 according to their docs:

• https://www.appveyor.com/docs/windows-images-software/#python

But the previous build was failing with:

Build started
git clone -q https://github.com/Pylons/pyramid_retry.git C:\projects\pyramid-retry
git fetch -q origin +refs/pull/22/merge:
git checkout -qf FETCH_HEAD
Running Install scripts
%PYTHON%\python.exe -m pip install -U pip
The system cannot find the path specified.
Command exited with code 1

https://ci.appveyor.com/project/Pylons/pyramid-retry/builds/38721755/job/b0tje2v4od4pvta0

So I'm not sure. Presumably this is still a net win for Windows users as it adds 3.7 and 3.8?

[jon-betts]

If you have time, there's also a very similar PR in to update pyramid_services too: mmerickel/pyramid_services#15