Replace Travis-CI with GitHub Actions

[althonos]

Type of changes

• CI
• Documentation
• Tests

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @PyFilesystem/maintainers may be pedantic in the code review.

Description

As discussed in #446, this PR replaces the old Travis-CI workflow with a new workflow that uses GitHub Actions. The new workflow will:

• test and mesure coverage on all supported Python versions:
  • CPython 2.7, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9
  • PyPy 2.7, 3.6, 3.7
• lint with the following tools:
  • pycodestyle
  • flake8
  • mypy
  • black
• upload coverage statistics to Coveralls.

I updated some parts of the code and documentation to make the code base pass the linting stage again, as we have been editing the code without paying attention to these for some time. I still need to update the CONTRIBUTING.md guide to introduce these requirements for anybody who would want to open a PR, since the CI will probably reject them at first.