move tests and sphinxext outside folder application code

[mattijn]

First attempt for #2788.
This also moves the sphinxext folder to the main folder.

All tests pass locally.

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description what happens in this PR.

• move and rename altair.examples to tests/examples_arguments_syntax
• move all tests within altair into tests folder, keeping the same folder structure how these test files appeared.
• move altair.sphinxext folder to sphinxext folder on main level
• run pytest functions against tests folder instead of altair folder
• add pytest-cov package to dependencies_dev.txt file, so the coverage functions also function in the Makefile
• change the function name iter_examples to iter_examples_arguments_syntax
• update the black and flake config pyproject.toml and setup.cfg to exclude the tests folder

[mattijn]

@binste, can you have a look if this is OK?

All tests now also pass on Github Actions including building the docs. I think this is a nice improvement.

Then after this we can merge the test-file changes from https://github.com/altair-viz/altair/pull/2770/files into a folder examples_methods_syntax on equal level of examples_arguments_syntax in the tests folder.

[binste]

Agree, this is a great improvement to the package structure, thanks @mattijn!

Could you also update the pytest commands in https://github.com/altair-viz/altair/blob/master/Makefile? Apart from that it looks good to me. I also built the documentation locally and all examples rendered fine.

[mattijn]

Thanks for the feedback @binste. I updated the makefile, I also had to add the pytest-cov package to get output for the coverage functions. All tests pass, merging.