Pip-tools: Add python 3.8 support

[atugushev]

What's the problem this feature will solve?

Python 3.8 is released, so it's time to support it.

Describe the solution you'd like

1. add "py37" env to tox.ini
2. remove 3.8-dev from .travis.yml
3. add "Programming Language :: Python :: 3.8" classifier to setup.py
4. add "3.8" dimension to .travis.yml (supported, see https://travis-ci.community/t/add-python-3-8-support/5463)
5. add "py37" dimension to .appveyor.yml (not supported yet, but will be on the nex image update, tracking issue: https://github.com/appveyor/ci/issues/3142)
6. add "3.8" to python-version list in .github/workflows/cron.yml (not supported yet, tracking issue: https://github.com/actions/setup-python/issues/30)

Alternative Solutions

N/A

Additional context

https://discuss.python.org/t/python-3-8-0-is-now-available/2478

[hramezani]

@atugushev I can work on this.

[atugushev]

Thanks, @hramezani! Appreciate it.

[hugovk]

🚀 Python 3.8 is now available on AppVeyor:

• https://github.com/appveyor/ci/issues/3142#issuecomment-552049896

[atugushev]

@hugovk Yeah! Finally, all opened issues are closed 😊