Adds GitHub Action to automatically publish to TestPyPI

[tomaroberts]

WIP

• GHA to push to TestPyPI on pull_request
• Uses setuptools_scm to dynamically adjust version number with every commit
  • nb: PyPI/TestPyPI don't like default version numbering in setuptools_scm, hence use local_scheme = "no-local-version" option
  • setuptools_scm creates _version.py with each commit and this is propagated through rest of code (such as CLI -v option, etc.)

[tomaroberts]

fetch-depth 0 was key to unlocking this PR working on GHAs. Without this, the checkout step in the GHA did not clone the repo sufficiently, therefore setuptools-scm thought the most recent tag was 0.1, hence created 0.1.dev1 version number every time, which caused the GHA to crash because when it pushed to TestPyPI, there was already an existing 0.1.dev1 version.

With fetch-depth 0, the GHA clones the repo such that it finds the latest tag, e.g. 0.1.2 and all the commits since then, so setuptools-scm is able to create a version label with accurate guessing, e.g. 0.1.2.dev37

This was v confusing on the whole...

See here for other users experiencing similar: https://github.com/pypa/setuptools_scm/issues/480

[tomaroberts]

Mild pain in the ass, but in the end...

• Adds setuptools-scm to dynamically set version
  • important: fetch-depth 0 to enable GitHub Action environment to see enough of repo history. Without this, setuptools-scm is not able to calculate version number properly as it lacks enough information.
• Adds GitHub Action to publish every committed version to TestPyPI with unique SemVer version number.
• Verifies latest TestPyPI build by creating new venv and installing latest version.
  • nb: 2.5 minute wait in GHA to allow TestPyPI to update, which enables verification of latest package version. Might be better to do this with a try statement. But this will do for now.