third-party test: Update Python versions

Drop 3.8. One of them (pyanalyze) already dropped support for 3.8, and
likely others will follow soon. I'd like to wait a bit to drop support
in typing-extensions itself, but I'm no longer interested in how 3.8
works in third-party packages.

Also add 3.12 and 3.13 for all of them. If any don't work, I'll drop
them again.

.github/workflows/third_party.yml

@@ -44,7 +44,7 @@ jobs:
         # PyPy is deliberately omitted here,
         # since pydantic's tests intermittently segfault on PyPy,
         # and it's nothing to do with typing_extensions
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -89,7 +89,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -137,7 +137,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -186,7 +186,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -235,7 +235,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -290,7 +290,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
@@ -340,7 +340,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps: