Use node-gyp compatible version of Python in runner for test workflow

The node-gyp dependency of this project uses a Python script.

Previously, the test GitHub Actions workflow was not configured to install a specific version of Python, so whichever
version of Python 3.x that was pre-installed on the GitHub Actions runner machine was used.

The documentation for the node-gyp@7.1.2 version used by this project indicates the newest supported Python version is
3.8. Clearly newer versions did work because the workflow has been running with Python 3.10. However, the macos-latest
runner was updated to using Python 3.11 and the script now fails when `npm install` is ran in the project:

ValueError: invalid mode: 'rU' while trying to load binding.

The solution is to install a specific version of Python. It seems safest to use the newest version explicitly stated as
supported by the node-gyp@7.1.2 documentation, so Python 3.8 is installed.

Author: per1234

.github/workflows/test-typescript-npm.yml

@@ -3,6 +3,10 @@ name: Test TypeScript
 env:
   # See: https://github.com/actions/setup-node/#readme
   NODE_VERSION: 10.x
+  # See: https://github.com/actions/setup-python/tree/main#available-versions-of-python
+  # Using newest version documented as supported by node-gyp dependency:
+  # https://github.com/nodejs/node-gyp/tree/v7.1.2#installation
+  PYTHON_VERSION: 3.8
 
 on:
   push:
@@ -60,6 +64,11 @@ jobs:
         with:
           node-version: ${{ env.NODE_VERSION }}
 
+      - name: Install Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
       - name: Install dependencies
         run: npm install