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]

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.