Some errors appear after install
When i execute:

```
aqt install -O qt 6.0.1 mac desktop  
```

I got:

```
2021-02-19 14:49:44,531 - aqt - INFO - aqtinstall(aqt) v1.1.1 on Python 3.9.1 [CPython Clang 12.0.0 (clang-1200.0.32.29)]
2021-02-19 14:50:04,901 - aqt - INFO - Patching qt/6.0.1/clang_64/bin/qmake
2021-02-19 14:50:04,920 - aqt - INFO - Finished installation
2021-02-19 14:50:04,920 - aqt - INFO - Time elasped: 20.39480686 second
/usr/local/Cellar/python@3.9/3.9.1_8/Frameworks/Python.framework/Versions/3.9/lib/python3.9/multiprocessing/resource_tracker.py:216: UserWarning: resource_tracker: There appear to be 4 leaked semaphore objects to clean up at shutdown     
  warnings.warn('resource_tracker: There appear to be %d '
```

How can i solve it?
diff --git a/aqt/installer.py b/aqt/installer.py
index 93076c2..f5d86f7 100644
--- a/aqt/installer.py
+++ b/aqt/installer.py
@@ -171,6 +171,8 @@ class Cli:
             tasks.append((arc, base_dir, sevenzip))
         pool = multiprocessing.Pool(self.settings.concurrency)
         pool.starmap(installer, tasks)
+        pool.close()
+        pool.join()
 
     def run_install(self, args):
         """Run install subcommand"""
['aqt/installer.py']