ci: allow mypyc job to fail

mypyc is not yet completely stable, and might fail for things that are
correct Python (see
!61 (comment 456440)),
so don't make it a requirement that it passes for merging

.gitlab-ci.yml

@@ -18,6 +18,7 @@ native-build:
     - adduser -D build
   script:
     - .ci/native-build.sh
+  allow_failure: true
 
 docs:
   stage: test