Declare a build dependency on `pip`

[AlexWaygood]

Currently installing fasttext with uv fails, as fasttext's setup.py file tries to use pip to install pybind11 if it is not already installed. uv does not, by default, include pip in new virtual environments:

% uv venv new-venv                 
Using Python 3.12.2 interpreter at: /Users/alexw/.pyenv/versions/3.12.2/bin/python3
Creating virtualenv at: new-venv
Activate with: source new-venv/bin/activate
% source new-venv/bin/activate     
(new-venv) % uv pip install fasttext
error: Failed to download and build: fasttext==0.9.2
  Caused by: Failed to build: fasttext==0.9.2
  Caused by: Build backend failed to determine extra requires with `build_wheel()`:
--- stdout:

--- stderr:
/Users/alexw/Library/Caches/uv/.tmp2ZDKtZ/.venv/bin/python: No module named pip
Traceback (most recent call last):
  File "<string>", line 38, in __init__
ModuleNotFoundError: No module named 'pybind11'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<string>", line 14, in <module>
  File "/Users/alexw/Library/Caches/uv/.tmp2ZDKtZ/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 325, in get_requires_for_build_wheel
    return self._get_build_requires(config_settings, requirements=['wheel'])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/alexw/Library/Caches/uv/.tmp2ZDKtZ/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 295, in _get_build_requires
    self.run_setup()
  File "/Users/alexw/Library/Caches/uv/.tmp2ZDKtZ/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 487, in run_setup
    super().run_setup(setup_script=setup_script)
  File "/Users/alexw/Library/Caches/uv/.tmp2ZDKtZ/.venv/lib/python3.12/site-packages/setuptools/build_meta.py", line 311, in run_setup
    exec(code, locals())
  File "<string>", line 72, in <module>
  File "<string>", line 41, in __init__
RuntimeError: pybind11 install failed.
---

This PR makes fasttext installable with uv by adding pip as a build dependency.

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