Fix CI failures due to removed runners in Github Actions (#1354)

The "ubuntu-20.04" runner was removed a few months ago, causing the
Loguru CI to repeatedly fail since then. On top of that, migrating to
"ubuntu-22.04" or "ubuntu-24.04" was not straightforward because these
runners were incompatible with "actions/setup-python" for Python 3.5 and
3.6.

As a workaround, I replaced "actions/setup-python" with a "python-slim"
Docker image for these versions that went unsupported. That's probably
the closest we can get to a stable and reproducible CI setup that won't
break overnight due to upstream decisions. ;)

Eventually, this solution will likely need to be repeated for other
Python versions as theyh become deprecated. I've considered defaulting
to a Docker image, but I assume the recommended approach is still
to favor "actions/setup-python" when it's available.

Additionally, because "actions/download-artifact" would not work within
the Docker image (conflicting GLIB versions between the OS and their
executable), I've moved the Codecov upload to a later stage in charge
of uploading all the coverage reports once the tests have been completed.

The downside is that we lose the "flags" that enabled us to identify
coverage reports individually on Codecov. But I don't know if we can do
otherwise.

Author: Delgan

.github/workflows/tests.yml

@@ -26,12 +26,14 @@ jobs:
         allow-failure:
         - false
         include:
-        - os: ubuntu-20.04
+        - os: ubuntu-22.04
           python-version: '3.5'
           allow-failure: false
-        - os: ubuntu-20.04
+          container: python:3.5-slim
+        - os: ubuntu-22.04
           python-version: '3.6'
           allow-failure: false
+          container: python:3.6-slim
         - os: windows-2022
           python-version: '3.12'
           allow-failure: false
@@ -42,16 +44,18 @@ jobs:
           python-version: 3.14-dev
           allow-failure: true
     runs-on: ${{ matrix.os }}
+    # Some versions of Python are not longer supported by GitHub Actions.
+    # For those, we use a container image and skip the setup-python step.
+    # It will be empty and ignored for the other matrix entries.
+    container: ${{ matrix.container }}
     steps:
     - name: Checkout repository
       uses: actions/checkout@v4
     - name: Set up Python
+      if: ${{ matrix.container == null }}
       uses: actions/setup-python@v5
       with:
         python-version: ${{ matrix.python-version }}
-      env:
-        # Workaround for https://github.com/actions/setup-python/issues/866
-        PIP_TRUSTED_HOST: pypi.python.org pypi.org files.pythonhosted.org
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
@@ -60,11 +64,23 @@ jobs:
       run: |
         tox -e tests
       continue-on-error: ${{ matrix.allow-failure }}
-    - name: Upload coverage to Codecov
+    - name: Upload coverage data
+      uses: actions/upload-artifact@v4
+      with:
+        name: coverage-${{ matrix.python-version }}-${{ matrix.os }}
+        path: coverage.xml
+      continue-on-error: ${{ matrix.allow-failure }}
+  push-coverage-to-codecov:
+    runs-on: ubuntu-22.04
+    needs: tests
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Download artifacts
+      uses: actions/download-artifact@v4
+    - name: Upload to Codecov
       uses: codecov/codecov-action@v5
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
-        flags: ${{ matrix.os }}_${{ matrix.python-version }}
         verbose: true
         fail_ci_if_error: true
-      continue-on-error: ${{ matrix.allow-failure }}