Issue 259 - Create GH Actions workflow to replace AppVeyor

[clyde-johnston]

This PR addresses issue #259 to move the AppVeyor workflow to GH Actions. Some differences:

• GHA workflow includes tests for Python 3.8 and 3.9 on Windows (these can be excluded if required)
• GHA workflow tests the demo, check_style and docs as graphviz is installed on the Windows runner (this makes Linux and Windows tests the same)

Important: The Windows self-hosted runners differ from the GH Actions runners in that they are not rebuilt after each workflow run. Therefore, there is a chance the Windows runners could become corrupted. A clean installation of the Windows runners can be made from AWS Images if required.

[clyde-johnston]

I have tested the PYPI_TEST_API_TOKEN_GLOBAL token on the testpypi repository and it works. I have amended the latest commit in this PR to use the PYPI_API_TOKEN_PYCYPHAL token on the pypi repository. This PR is ready to be merged.

Note: The GHA workflow will fail to deploy until the pycyphal version number is incremented.

[clyde-johnston]

The pull request event was added to the workflow as requested.

[clyde-johnston]

Note: The demo test fails on Ubuntu runners occasionally and randomly with a KeyboardInterrupt error. A rerun of the job usually resolves this.

[pavel-kirienko]

Could you please rename the workflow file so that the name indicates that it also runs the test suite aside from publishing the release?

[coveralls]

coverage: 95.357% (+0.005%) from 95.352% when pulling c56d148 on issue-259 into 01b9a9b on master.

[clyde-johnston]

Sonar and Coveralls are now working. This PR is ready for review.

[pavel-kirienko]

This looks good but please confirm that the exit status is not ignored on Windows (this issue has been observed in Yakut). Feel free to merge after this is verified/addressed. Thanks!