Migrate to GitHub Actions CI and resolve dependency issues

[tw4l]

Fixes #120

• Migrates from Travis CI to GitHub Actions CI
• Tests Python versions 3.7-3.11
• Updates httpbin to latest PSF-maintained version
• Modifies handling of proxies in test_capture_http_proxy module to account for breaking changes in requests and urllib3 (note that this currently means pinning urllib3==1.25.11, as more recent versions no longer allow using http:// scheme URLs with the https key in the proxies dictionary; depending on whether and how this is resolved, we may need to come back to this in the future but for now it gets CI working again)

There are some unresolved deprecation warnings that would be best handled in a separate issue/PR, particularly around using setup.py test, which we'll likely want to resolve by switching to tox as a test runner.

[tw4l]

Very good point! I've added an issue for that: https://github.com/webrecorder/warcio/issues/165

I'll go ahead and merge this for now so that we can get CI running again for outstanding PRs.