Switch to github actions

[matham]

I recently switched the base flower and some actual flowers to use github actions. I'm opening an issue here to suggest this flower also update if you can.

You can see the changes I made here: https://github.com/kivy-garden/graph/commits/master.

With these changes, the action will:

• Run tests on win/linux and lint (PEP8) the code for each push/PR.
• Auto generate wheels and source dist for each push/PR as artifacts.
• On a tag it'll also create a draft release on github with all the wheels. This will make it easier to manually upload to pypi.
• On each push it'll generate the docs and upload to gh-pages.

The following changes were also made in this PR and previously:

• setup.py uses find_namespace_packages. extras_require was updated.
• We switched to a _version.py file because otherwise there are import issues if kivy is not already pre-installed (Externalize __version__ flower#1).
• For the docs, we switched to the sphinx_rtd_theme theme in conf.py.
• I moved the tests directory under kivy_garden/flower so it is installed when the flower is installed.
• Removed .travis.yml and added github workflow.