Enclose ThreadPool.map call in a try/finally block

lib/spack/spack/compilers/__init__.py

@@ -210,7 +210,9 @@ def find_compilers(*path_hints):
 
     # Here we map the function arguments to the corresponding calls
     tp = multiprocessing.pool.ThreadPool()
+    try:
         detected_versions = tp.map(detect_version, arguments)
+    finally:
         tp.close()
 
     def valid_version(item):