Publish to PyPI via github action

[c24t]

Fixes #8

This PR adds a GH action that builds all packages in the repo on each release and pushes them to PyPI.

The build script was inspired by OC's twine_upload.sh. This version only builds the wheels, and leaves it up to the GH action to publish to PyPI.

I added a new PyPI API token for user cnk to this project's secrets.

See also #167, which added the first GH action to publish docs.

[c24t]

Replaced https://github.com/pypa/gh-action-pypi-publish with the twine invocation from the python-publish starter workflow.

[Oberon00]

Shouldn't we use pip wheel instead of invoking setup.py directly?

[toumorokoshi]

seems like it just calls setup.py under the hood. Not sure why we'd want to abstract ourselves away a layer: https://pip.pypa.io/en/stable/reference/pip_wheel/#build-system-interface

[Oberon00]

I couldn't find any guidance on whether pip wheel or setup.py bdist_wheel is preferred. I know that pip install is preferred over setup.py install but I guess the arguments there don't apply to pip wheel. So I think it's fine either way.

[Oberon00]

Looking at https://github.com/pypa/pip/blob/master/src/pip/_internal/commands/wheel.py, I think it's safe to say that pip wheel just calls setup.py is a gross oversimplification. But if we need all the extra work that pip does is a whole other question.

What would be useful is generating a source distribution first using setup.py and then generate a wheel from that using pip wheel. That would verify that our MANIFEST.in, etc. is working correctly.

[c24t]

I added #252 for building a source distribution first. I'll have to dig into the docs to develop an opinion here.

[c24t]

One complication to note here is that pip wheel isolates builds from the env.

setup.py bdist_wheel builds as expected:

$ (cd opentelemetry-sdk/; python3 setup.py bdist_wheel --dist-dir='../dist' sdist --dist-dir='../dist'); ls dist

...
opentelemetry-sdk-0.4.dev0.tar.gz  opentelemetry_sdk-0.4.dev0-py3-none-any.whl

pip wheel fails because it tries to pip-install dependencies instead of using the current env:

(cd opentelemetry-sdk/; python3 setup.py sdist --dist-dir='../dist'); (cd dist; pip wheel opentelemetry-sdk-0.4.dev0.tar.gz); ls dist

...
Processing ./opentelemetry-sdk-0.4.dev0.tar.gz
ERROR: Could not find a version that satisfies the requirement opentelemetry-api==0.4.dev0 (from opentelemetry-sdk==0.4.dev0) (from versions: 0.1a0, 0.2a0, 0.3a0)
ERROR: No matching distribution found for opentelemetry-api==0.4.dev0 (from opentelemetry-sdk==0.4.dev0)
opentelemetry-sdk-0.4.dev0.tar.gz

[toumorokoshi]

I have a couple relevant comments in #248 (sorry to comment on the WIP ticket), but no blockers. Nice call on automated releases!

[c24t]

@toumorokoshi's comment from the testing PR, https://github.com/open-telemetry/opentelemetry-python/pull/248/files#r340186555:

I think we should add universal here, so that package will resolve on all platforms:

https://packaging.python.org/guides/distributing-packages-using-setuptools/#universal-wheels

Or we could go modify everyone's setup.cfg, but that feels duplicative and error prone.

Don't universal wheels need to be compatible with python 2? Our releases are tagged py3-none-any: python3 only, no ABI, any platform (from PEP 0427).