MNT Uses github actions for uploading to pypi

[thomasjpfan]

This PR adds a github action so we can upload with pypi through the interface.

1. One needs to add the PYPI_TOKEN secret to enable uploading to pypi. (I already added a TEST_PYPI_TOKEN for testpypi).
2. One has to type in pypi in the "Run workflow" UI to upload to pypi, otherwise it will upload to testpypi by default.
3. There is a check_artifacts.py script that makes sure that wheelhouse_uploader fetches the correctly number of artifacts based on the azure-pipeline.yml configuration.

CC @ogrisel

[adrinjalali]

Do we now also fetch the sditst? I remember that was done manually

[thomasjpfan]

Do we now also fetch the sditst? I remember that was done manually

With ogrisel/wheelhouse-uploader#36 this also fetches the sdist, which is currently built in azure-pipelines.yml.

Here is an example for the output of the worflow, where you can see the sdist being downloaded. The overall workflow expectingly fails because testpypi already has the version 0.23.2 uploaded.

[ogrisel]

Now that we moved to build scikit-learn wheels directly in the main repo, this would have to be adapted to add it there instead:

https://github.com/scikit-learn/scikit-learn/blob/master/.github/workflows/wheels.yml

/cc @alfaro96