feat: push releases to pypi

[xgui3783]

This PR:

• add some of the missing tests for nifti rgb support
• adds workflow that automatically builds and pushes releases to pypi

Before this PR should be merged...

please:

• check if the build step:

python -m pip install -U pip
python -m pip install --user --upgrade twine setuptools wheel
python setup.py sdist bdist_wheel

and upload step:

python -m twine upload dist/*

are correct.

• create a token (log into pypi > [USERNAME] > Your projects > neuroglancer-scripts [manage] > Settings > Create a token for neuroglancer-scripts )
• Add the said token as as a secret with the name PYPI_PUSH in github (Settings > Secrets > New repository secret)

(It should be noted that the token, once generated, and saved, cannot be viewed in plain text by any one. Should a token be lost, it can always be revoked at pypi and a new one generated.)

[ylep]

Thanks a lot @xgui3783 for this promising PR, I will give it a look after I return from vacation on 2nd November!