decouple pyproject.toml from the test suite

decouple pyproject.toml from the test suite

tools/write_pyi.py is run as part of the test suite,
which has to run from a source distribution, which
does not have pyproject.toml right now due to
issues reported w/ pip.  these pip issues are probably
resolved now however we have not gone back to including
pyproject.toml at the moment.   so we therefore have to
hardcode the single "-l79" black config option in
write_pyi.py.

Change-Id: I877f69eb2322f6cbb39ed4c6d4a9adf598703223