aider: Added a test function to check if the pretty attribute of the InputOutput class is set to False when the NO_COLOR environment variable is set.

# Context:
USER: add a test for NO_COLOR
ASSISTANT: To add a test for the `NO_COLOR` environment variable, we will create a new test function in the `tests/test_io.py` file. Since this file does not exist, we will create it first.

```python
tests/test_io.py
<<<<<<< ORIGINAL
=======
import os
import unittest
from unittest.mock import patch
from aider.io import InputOutput

class TestInputOutput(unittest.TestCase):
    def test_no_color_environment_variable(self):
        with patch.dict(os.environ, {"NO_COLOR": "1"}):
            io = InputOutput()
            self.assertFalse(io.pretty)

if __name__ == "__main__":
    unittest.main()
>>>>>>> UPDATED
```

This test function checks if the `pretty` attribute of the `InputOutput` class is set to `False` when the `NO_COLOR` environment variable is set.

tests/test_io.py

@@ -0,0 +1,13 @@
+import os
+import unittest
+from unittest.mock import patch
+from aider.io import InputOutput
+
+class TestInputOutput(unittest.TestCase):
+    def test_no_color_environment_variable(self):
+        with patch.dict(os.environ, {"NO_COLOR": "1"}):
+            io = InputOutput()
+            self.assertFalse(io.pretty)
+
+if __name__ == "__main__":
+    unittest.main()