Replace Travis CI with GitHub Actions

[hugovk]

Travis CI has a new pricing model which places limits on open source.

• https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing
• https://www.jeffgeerling.com/blog/2020/travis-cis-new-pricing-plan-threw-wrench-my-open-source-works

Travis support has also become unresponsive to developers who have run out of free credits, and many projects are moving to GitHub Actions instead.

And https://github.com/pypa builds can not longer build:

Builds have been temporarily disabled for public repositories due to a negative credit balance. Please go to the Plan page to replenish your credit balance or alter your Consume paid credits for OSS setting.

Owner pypa does not have enough credits.

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This GHA config does much the same as the deleted Travis one. Two differences: tests with current newest Python 3.9 instead of 3.6; and adds caching of pip downloads, to try and help ease the load on PyPI, and speed up builds a bit.

Sample build:

• https://github.com/hugovk/packaging.python.org/runs/1822833000?check_suite_focus=true

[webknjaz]

FWIW this PR isn't urgent because we have an RTD-based check in PRs that essentially does the same smoke-test. Other than that, I fully support this effort.