"dataclasses" missing in requirements.txt

After downloading mesonwrap and installing all Python packages listed in requirements.txt, ``mesonwrap`` failed with:

"ModuleNotFoundError: No module named dataclasses."

Assuming that requirements.txt lists all required Python packages, ``dataclasses`` should be listed there.

Start by inspecting requirements.txt and reproduce the reported installation failure with the listed Python packages. Add the missing dataclasses dependency there, then verify that installing from requirements.txt allows mesonwrap to import successfully.

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