[Python distribtests] Fix version issue for readme_renderer (grpc#34316)

Distribtests is failing with the following error:
```
Collecting twine<=2.0
  Downloading twine-2.0.0-py3-none-any.whl (34 kB)
Collecting pkginfo>=1.4.2 (from twine<=2.0)
  Downloading pkginfo-1.9.6-py3-none-any.whl (30 kB)
Collecting readme-renderer>=21.0 (from twine<=2.0)
  Obtaining dependency information for readme-renderer>=21.0 from https://files.pythonhosted.org/packages/b5/7e/992e0e21b37cadd668226f75fef0aa81bf21c2426c98bc06a55e514cb323/readme_renderer-42.0-py3-none-any.whl.metadata
  Downloading readme_renderer-42.0-py3-none-any.whl.metadata (2.8 kB)
Collecting requests>=2.20 (from twine<=2.0)
  Obtaining dependency information for requests>=2.20 from https://files.pythonhosted.org/packages/70/8e/0e2d847013cb52cd35b38c009bb167a1a26b2ce6cd6965bf26b47bc0bf44/requests-2.31.0-py3-none-any.whl.metadata
  Downloading requests-2.31.0-py3-none-any.whl.metadata (4.6 kB)
Collecting requests-toolbelt!=0.9.0,>=0.8.0 (from twine<=2.0)
  Downloading requests_toolbelt-1.0.0-py2.py3-none-any.whl (54 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 54.5/54.5 kB 5.1 MB/s eta 0:00:00
Requirement already satisfied: setuptools>=0.7.0 in ./venv/lib/python3.9/site-packages (from twine<=2.0) (68.2.0)
Collecting tqdm>=4.14 (from twine<=2.0)
  Obtaining dependency information for tqdm>=4.14 from https://files.pythonhosted.org/packages/00/e5/f12a80907d0884e6dff9c16d0c0114d81b8cd07dc3ae54c5e962cc83037e/tqdm-4.66.1-py3-none-any.whl.metadata
  Downloading tqdm-4.66.1-py3-none-any.whl.metadata (57 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 57.6/57.6 kB 7.6 MB/s eta 0:00:00
Collecting nh3>=0.2.14 (from readme-renderer>=21.0->twine<=2.0)
  Downloading nh3-0.2.14.tar.gz (14 kB)
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
  Preparing metadata (pyproject.toml): started
  Preparing metadata (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error

  × Preparing metadata (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [6 lines of output]

      Cargo, the Rust package manager, is not installed or is not on PATH.
      This package requires Rust and Cargo to compile extensions. Install it through
      the system's package manager or via https://rustup.rs/

      Checking for Rust toolchain....
      [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.
```

### Why

* We're pulling readme_renderer 42.0 from twine, since 42.0 requires nh3
and nh3 requires Rust, the test is failing.

### Fix

* Pinged readme_renderer to `<40.0` since any version higher or equal to
40.0 requires Python 3.8.

### Testing

* Passed manual run: http://sponge/57d815a7-629f-455f-b710-5b80369206cd
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tools/run_tests/artifacts/build_artifact_python.sh

@@ -155,7 +155,7 @@ then
   "${PYTHON}" -m pip install virtualenv
   "${PYTHON}" -m virtualenv venv || { "${PYTHON}" -m pip install virtualenv==20.0.23 && "${PYTHON}" -m virtualenv venv; }
   # Ensure the generated artifacts are valid using "twine check"
-  venv/bin/python -m pip install "twine<=2.0"
+  venv/bin/python -m pip install "twine<=2.0" "readme_renderer<40.0"
   venv/bin/python -m twine check dist/* tools/distrib/python/grpcio_tools/dist/*
   rm -rf venv/
 fi