Pin gpt_index version

This MR pins gpt_index (was renamed to llama_index) version because currently pip install -r requirements.txt fails with

$ pip install -r requirements.txt
Obtaining file:///Users/dgruzd/projects/ask-the-tanuki (from -r requirements.txt (line 1))
  Preparing metadata (setup.py) ... error
  error: subprocess-exited-with-error

  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [7 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/Users/dgruzd/projects/ask-the-tanuki/setup.py", line 7, in <module>
          with open(Path(__file__).absolute().parents[0] / "gpt_index" / "VERSION") as _f:
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
      FileNotFoundError: [Errno 2] No such file or directory: '/Users/dgruzd/projects/ask-the-tanuki/gpt_index/VERSION'
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.