Update Python versions in "pytest" workflow

- Add Python 3.13
- Drop Python 3.8

.github/workflows/pytest.yaml

@@ -24,11 +24,11 @@ jobs:
         - ubuntu-latest
         - windows-latest
         python-version:
-        - "3.8"  # Earliest version supported by ixmp
-        - "3.9"
+        - "3.9"  # Earliest version supported by ixmp
         - "3.10"
         - "3.11"
-        - "3.12"  # Latest supported by ixmp
+        - "3.12"
+        - "3.13.0-rc.2"  # Latest supported by ixmp
         gams-version:
         # Version used until 2024-07; disabled
         # - 25.1.1
@@ -46,12 +46,12 @@ jobs:
         # No arm64 distribution for this version of GAMS
         # - { os: macos-latest, gams-version: 25.1.1}
         # No arm64 distributions of JPype for these Pythons
-        - { os: macos-latest, python-version: "3.8" }
         - { os: macos-latest, python-version: "3.9" }
         # Redundant with macos-latest
         - { os: macos-13, python-version: "3.10" }
         - { os: macos-13, python-version: "3.11" }
         - { os: macos-13, python-version: "3.12" }
+        - { os: macos-13, python-version: "3.13.0-rc.2" }
         # Example: pandas 2.0 requires Python >= 3.8
         # - { python-version: "3.7", pandas-version: "==2.0.0rc0" }
 
@@ -81,7 +81,7 @@ jobs:
     - uses: r-lib/actions/setup-r@v2
       id: setup-r
       with:
-        r-version: '4.2.3'
+        r-version: "4.2.3"
 
     - name: Cache GAMS installer and R packages
       uses: actions/cache@v4