Add GitHub Actions workflow to run all python tests

[claremacrae]

This adds a GitHub Actions workflow, in a separate file, .github/workflows/github_actions_build.yml, to set up an environment and then run all our python tests, of both the release-building scripts and the Sphinx documentation-building scripts.

It also adds a build-badge for this new workflow.

[sourcery-ai[bot]]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.08%.

Quality metrics	Before	After	Change
Complexity	0.43 ⭐	0.43 ⭐	0.00
Method Length	22.29 ⭐	21.71 ⭐	-0.58 👍
Working memory	5.14 ⭐	5.14 ⭐	0.00
Quality	92.00% ⭐	92.08% ⭐	0.08% 👍

Other metrics	Before	After	Change
Lines	53	54	1

Changed files	Quality Before	Quality After	Quality Change
doc/sphinx/tests/test_markdown_conversion.py	92.00% ⭐	92.08% ⭐	0.08% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation

Legend and Explanation

The emojis denote the absolute quality of the code:

• ⭐ excellent
• 🙂 good
• 😞 poor
• ⛔ very poor

The 👍 and 👎 indicate whether the quality has improved or gotten worse with this pull request.

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Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it by mentioning @sourcery-ai in a comment.