Allow pyproject plugin to work with python < 3.11

Stuart Prescottrequested to merge
stuart/dh-python:tmp/pyproj-toml into master

Reliance on tomllib from stdlib makes the pyproject plugin fail with older python interpreters — this would prevent backporting dh-python to Debian oldstable or LTS releases of various derivatives.

Related to: #1032866 Closes: #1043301

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