Update custom component installation requirements

[shimizust]

Description

• Added pip 21.3+ requirement to be able to build from pyproject.toml otherwise get errors of this type:

 👷 Installing python... (/my/path/.venv/bin/pip3 install -e testcomponent[dev])                                                                           │
│ 🟥 Python installation failed                                                                                                                                                                                  │
│ ERROR: File "setup.py" or "setup.cfg" not found. Directory cannot be installed in editable mode: /my/path/gradio-components/testcomponent              │
│ (A "pyproject.toml" file was found, but editable mode currently requires a setuptools-based build.) 

[gradio-pr-bot]

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[abidlabs]

Thank you @shimizust for adding this clarification