ci: migrate workflows to Gitea Actions

ci/python-tests.sh

@@ -1,5 +1,6 @@
 #!/bin/sh
 # Run tests in an isolated, disposable environment on a Linux Actions runner.
+# Gitea Actions supplies the GitHub-compatible GITHUB_* runner variables.
 set -eu
 
 fail() {
@@ -13,7 +14,7 @@ fail() {
 : "${GITHUB_JOB:?GITHUB_JOB is required}"
 for run_identifier in "$GITHUB_RUN_ID" "$GITHUB_RUN_ATTEMPT"; do
     case "$run_identifier" in
-        ''|*[!0-9]*) fail 'GitHub run ID and attempt must be numeric' ;;
+        ''|*[!0-9]*) fail 'Actions run ID and attempt must be numeric' ;;
     esac
 done
 case "$GITHUB_JOB" in