Python build fails with latest version of maturin

ref: https://www.maturin.rs/migration.html#from-014-to-015

build fails with error:

💥 maturin failed
  Caused by: pyproject.toml is invalid
  Caused by: pyproject.toml is not PEP 517 compliant: TOML parse error at line 1, column 1
  |
1 | [build-system]
  | ^
invalid type: sequence, expected string or map