feat: add GitHub Actions workflow to run pytest

- sets Python version for GitHub workflows
- adds workflow and shell script that runs pytest

.github/workflows/.python-version

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+3.12

.github/workflows/tests-pytest.yml

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+name: Pytest
+
+on: [push, pull_request, workflow_call]
+
+jobs:
+  pytest:
+    runs-on: ubuntu-latest
+    permissions:
+      # Gives the action the necessary permissions for publishing new
+      # comments in pull requests.
+      pull-requests: write
+      # Gives the action the necessary permissions for pushing data to the
+      # python-coverage-comment-action branch, and for editing existing
+      # comments (to avoid publishing multiple comments in the same PR)
+      contents: write
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Install system packages
+        run: |
+          sudo apt-get update -y
+          sudo apt-get install -y gettext
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version-file: .github/workflows/.python-version
+          cache: pip
+          cache-dependency-path: "**/pyproject.toml"
+
+      - name: Install Python dependencies
+        run: pip install -e .[test]
+
+      - name: Run setup
+        run: ./bin/init.sh
+
+      - name: Run tests
+        run: ./tests/pytest/run.sh
+
+      - name: Upload coverage report
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: pems/static/coverage
+
+      - name: Coverage comment
+        uses: py-cov-action/python-coverage-comment-action@v3
+        with:
+          GITHUB_TOKEN: ${{ github.token }}
+          MINIMUM_GREEN: 90
+          MINIMUM_ORANGE: 80

tests/pytest/run.sh

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+#!/usr/bin/env bash
+set -eu
+
+# run normal pytests
+coverage run -m pytest
+
+# clean out old coverage results
+rm -rf pems/static/coverage
+
+coverage html --directory pems/static/coverage