tests: check min required python version with vermin (#14289)

This commit removes the `python_version.py` unit test module
and the vendored dependencies `pyqver2.py` and `pyqver3.py`.
It substitutes them with an equivalent check done using
`vermin` that is run as a separate workflow via Github Actions.

This allows us to delete 2 vendored dependencies that are unmaintained
and substitutes them with a maintained tool.

Also, updates the list of vendored dependencies.

.github/workflows/minimum_python_versions.yaml

@@ -0,0 +1,30 @@
+name: Minimum Python Versions
+
+on:
+  push:
+    branches:
+      - master
+      - develop
+  pull_request:
+    branches:
+      - master
+      - develop
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v1
+    - name: Setup Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 2.7
+    - name: Install Python Packages
+      run: |
+        pip install --upgrade pip
+        pip install --upgrade vermin
+    - name: Minimum Version (Spack's Core)
+      run: vermin --backport argparse -t=2.6- -t=3.5- -v lib/spack/spack/ lib/spack/llnl/ bin/
+    - name: Minimum Version (Repositories)
+      run: vermin --backport argparse -t=2.6- -t=3.5- -v var/spack/repos