CI: Support Python 3.6

.github/workflows/main.yml

@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: ["ubuntu-latest"]
+        os: ["ubuntu-20.04"]
         python-version: ["3.6", "3.12"]
 
     env:
@@ -43,19 +43,30 @@ jobs:
         cache: 'pip'
         cache-dependency-path: 'pyproject.toml'
 
-    - name: Set up project
+    - name: Set up project (Python 3.6)
+      if: matrix.python-version == '3.6' || matrix.python-version == 'pypy-3.6'
       run: |
+        # Install requirements for testing.
+        pip install --requirement requirements-test.txt
 
+    - name: Set up project (Python 3.7+)
+      if: matrix.python-version != '3.6' && matrix.python-version != 'pypy-3.6'
+      run: |
         # `setuptools 0.64.0` adds support for editable install hooks (PEP 660).
         # https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6400
         pip install "setuptools>=64" --upgrade
 
         # Install package in editable mode.
         pip install --use-pep517 --prefer-binary --editable=.[develop,test]
 
-    - name: Run linter and software tests
+    - name: Check code style
+      if: matrix.python-version != '3.6' && matrix.python-version != 'pypy-3.6'
+      run: |
+        poe lint
+
+    - name: Run software tests
       run: |
-        poe check
+        poe test
 
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v3

requirements-test.txt

@@ -0,0 +1,7 @@
+# Needed for testing on Python 3.6
+
+# tests
+poethepoet<1
+pretend<2
+pytest<8
+pytest-cov<5