ci: add the option to test more Python versions

It seems like a good idea to be able to test the lowest version we support. And
there have been enougth issue with 3.12 that we need to be able to make sur we
do not break it. We should probably get a matrix setup for more version and
flavor, but that is a simple and efficient start.

contrib/heptapod-ci.yml

@@ -66,7 +66,7 rust-cargo-test:
     variables:
         CI_CLEVER_CLOUD_FLAVOR: S
 
-test-c:
+test-c: &test_c
     <<: *runtests
     variables:
         RUNTEST_ARGS: " --no-rust --blacklist /tmp/check-tests.txt"
@@ -99,6 +99,21 test-chg:
         RUNTEST_ARGS: "--blacklist /tmp/check-tests.txt --chg"
         TEST_HGMODULEPOLICY: "c"
 
+# note: we should probably get a full matrix for flavor × py-version, but this
+# is a simple start to be able to check if we break the lowest supported
+# version (and 3.12 have been giving us various troubles)
+test-3.8-c:
+    <<: *test_c
+    when: manual  # avoid overloading the CI by default
+    variables:
+        PYTHON: python3.8
+
+test-3.12-c:
+    <<: *test_c
+    when: manual  # avoid overloading the CI by default
+    variables:
+        PYTHON: python3.12
+
 check-pytype:
     extends: .runtests_template
     before_script: