aider: Replaced usage of redirect_stdin with closing stdin before calling main.

# Context:
USER: ImportError: cannot import name 'redirect_stdin' from 'contextlib'. Maybe just close stdin before calling main?
ASSISTANT: To fix the ImportError, we can replace the usage of `redirect_stdin` with closing stdin before calling `main`. Here's the change:

```python
tests/test_main.py
<<<<<<< ORIGINAL
from contextlib import redirect_stdin
=======
>>>>>>> UPDATED
```

```python
tests/test_main.py
<<<<<<< ORIGINAL
    with open(os.devnull, 'r') as dev_null:
        with redirect_stdin(dev_null):
                    main()
=======
    with open(os.devnull, 'r') as dev_null:
        sys.stdin.close()
        main()
>>>>>>> UPDATED
```

tests/test_main.py

@@ -1,7 +1,6 @@
 import os
 import sys
 import tempfile
-from contextlib import redirect_stdin
 from unittest import TestCase
 from aider.main import main
 
@@ -10,5 +9,5 @@ class TestMain(TestCase):
         with tempfile.TemporaryDirectory() as temp_dir:
             os.chdir(temp_dir)
     with open(os.devnull, 'r') as dev_null:
-        with redirect_stdin(dev_null):
+        sys.stdin.close()
         main()