fix: fixed string for constraints - see issue #58 (#64)

There is no test - yet. I am not sure how to test constraints in the
dummy SLURM setup of the CI.

snakemake_executor_plugin_slurm/__init__.py

@@ -151,7 +151,7 @@ def run_job(self, job: JobExecutorInterface):
             )
 
         if job.resources.get("constraint"):
-            call += f" -C {job.resources.constraint}"
+            call += f" -C '{job.resources.constraint}'"
         if job.resources.get("mem_mb_per_cpu"):
             call += f" --mem-per-cpu {job.resources.mem_mb_per_cpu}"
         elif job.resources.get("mem_mb"):