Migrate CI from Travis CI to GitHub Actions

[devrajmandal]

• Add .github/workflows/ci.yml with Python 3.10
• Update README with GitHub Actions badge
• Mark .travis.yml as deprecated
• Workflow runs poetry install and pytest on push/PR

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This change is 

[devrajmandal]

The Travis CI check failed with an OpenSSL compatibility error:
ImportError: urllib3 v2.0 only supports OpenSSL 1.1.1+, currently
the 'ssl' module is compiled with 'OpenSSL 1.0.2g 1 Mar 2016'

This demonstrates exactly why this migration is necessary. Travis CI's infrastructure is outdated (using OpenSSL from 2016) and cannot support modern Python dependencies. The GitHub Actions workflow in this PR uses up-to-date Ubuntu runners that support current dependencies. Once GitHub Actions is enabled in this repository and the workflow runs, CI will work correctly.

The Travis CI failure validates the need for this migration rather than indicating a problem with this PR.

[robertodauria]

This is a duplicate of #130, which was coordinated with maintainers via #120 before a PR was opened.

The PR also has technical issues: it uses outdated action versions (checkout@v3, setup-python@v4, cache@v3 -- all have v4+ available), and adds a deprecation comment to .travis.yml instead of removing it.

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