text: Title: Test failure after initial setup for development
  Body: ### System Info
  
  OS version: Windows 10 Pro
  Python version: 3.10.10
  PandasAI version: 1.3.3
  
  ### 🐛 Describe the bug
  
  After a clean setup for initial development, notice that 2 of the tests are failing:
  
  ```bash
  ================================================================================ short test summary info ================================================================================= 
  FAILED tests/test_smartdataframe.py::TestSmartDataframe::test_save_chart_non_default_dir - AssertionError: assert 'exports/char...b180ab5a5.png' == 'exports/char...b180ab5a5.png'
  FAILED tests/callbacks/test_file.py::TestFileCallback::test_on_code - PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\Henrique\\Desktop\\area de trabalho\\pessoal\\repos\\pandas...
  ================================================================= 2 failed, 248 passed, 2 skipped, 40 warnings in 12.09s =================================================================
  ```
  
  I'll give extra guidance on both errors.
  
  ## `test_on_code`
  This is returning `none` for the `self.assertEqual(contents, response)` statements, although the strings compared are equal.
  
  And why are mixed frameworks for testing? There are some tests using the built-in `unittest` module. Shouldn't it be standardized all to `pytest`?
  
  ## `test_save_chart_non_default_dir`
  
  This may have a connection with the OS. The test is failing on the following assertion:
  ```python
          assert (
              plt_mock.savefig.call_args.args[0]
              == f"exports/charts/{smart_dataframe.last_prompt_id}.png"
          )
  ```
  
  This is the best practice because the path is fixed for Unix-style systems. The problem is precisely in the fixed path variable.
  
  ```python
  plt_mock.savefig.call_args.args[0] = 'exports/charts\\cf69927e-2ec6-4ae6-a063-0d3d30598328.png'
  f"exports/charts/{smart_dataframe.last_prompt_id}.png" = 'exports/charts/cf69927e-2ec6-4ae6-a063-0d3d30598328.png'
  ```
  
  @gventuri, would you like to let someone work on this? Or can I handle it? I prefer to let new contributors have a chance to work on it, especially the second one, which is a path and OS related problem.