Test lower limits of dependencies

.ci_support/environment-old.yml

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+channels:
+  - conda-forge
+dependencies:
+  - lammps =2022.06.23
+  - openmpi
+  - numpy =1.23.5
+  - mpi4py =3.1.4
+  - pympipool =0.7.0
+  - ase =3.20.1
+  - scipy =1.9.3

.github/workflows/unittests-old.yml

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+# This workflow will install Python dependencies, run tests and lint with a variety of Python versions
+# For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
+
+name: Unittest Lower Bound
+
+on:
+  push:
+    branches: [ main ]
+  pull_request:
+    branches: [ main ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: conda-incubator/setup-miniconda@v2
+      with:
+        python-version: '3.9'
+        miniforge-variant: Mambaforge
+        channels: conda-forge
+        channel-priority: strict
+        use-mamba: true
+        environment-file: .ci_support/environment-old.yml
+    - name: Setup
+      shell: bash -l {0}
+      run: pip install --no-deps .
+    - name: Test
+      shell: bash -l {0}
+      timeout-minutes: 5
+      run: python -m unittest discover tests