Add support for Python 3.8

[hugovk]

What's the problem this feature will solve?

Add support for the newest version of Python, released 2019-10-14.

Describe the solution you'd like

Add metadata and testing for Python 3.8, and any necessary code changes.

Alternative Solutions

Don't support Python 3.8 (!)

Additional context

Checklist:

• Metadata: add Trove classifier, update README: PR Declare support for Python 3.8 #7219
• AppVeyor: pending Python 3.8.0 appveyor/ci#3142 - removed AppVeyor in Remove AppVeyor #7564
• Azure Pipelines: pending microsoft/azure-pipelines-image-generation#1317 Azure Pipelines: Test Windows on Python 3.8 #7320, Azure Pipelines: Test Linux and macOS on Python 3.8 #7321
• Travis CI: switch 3.8-dev to 3.8 and test earlier: PR Test on Python 3.8 final #7224
• Travis CI: as above and also change base Python to 3.8: PR Travis CI: Test Python 3.8 #7221. Currently blocked on Add Python 3.8 support actions/setup-python#30 (so the docs build can have 1 Python mentioned in config).

[hugovk]

Python 3.8 is now available on Azure Pipelines.

• https://github.com/microsoft/azure-pipelines-image-generation/issues/1317

Please see:

• PR Azure Pipelines: Test Windows on Python 3.8 #7320 for Windows (currently failing) and
• PR Azure Pipelines: Test Linux and macOS on Python 3.8 #7321 for Linux and macOS (split from failing Windows to allow merge sooner)

[hugovk]

• AppVeyor: pending appveyor/ci#3142

Python 3.8 is now available on AppVeyor.

• Python 3.8.0 appveyor/ci#3142 (comment)

Right now only 2.7 and 3.6 are tested on AppVeyor. Would you also like to test 3.8 on AppVeyor or only Azure Pipelines?

[pradyunsg]

IMO:

• Add 3.6 to Azure.
• Drop 3.6 from AppVeyor.
• Add 3.8 on AppVeyor.

[hugovk]

🚀 All checkboxes are now checked, closing!