Description of the desired feature

We recently migrated all the CI testings and style checks from Azure Pipelines to GitHub Actions. Currently, we're using both GitHub Actions and Travis CI.

The Travis CI is doing following jobs:

• Test PyGMT with Python 3.6-3.8 on Linux
• Deploy documentation to gh-pages branch for master branch and tags
• Publish PyGMT to PyPi for tags (Migrate PyPI release to GitHub Actions #679)

The first one is already covered in the GitHub Actions, so it's OK to just remove them from Travis CI.

The 2nd and the 3rd one should be migrated to GitHub Actions.

Are you willing to help implement and maintain this feature? Yes, but contributions are welcome.