CoverageException is fine as a base class, but not good to use for
raising (and catching sometimes).  Introduce specialized exceptions that
allow third-party tools to integrate better.

tomli couldn't use coverage themselves because we imported it early.
Cleaning sys.modules means their own imports will actually execute after
coverage has started, so their files will be properly measured.

This makes exceptions report their causes correctly, as "The above exception was
the direct cause of the following exception" instead of "During handling of the
above exception, another exception occurred."

* Support TOML v1.0.0 syntax in `pyproject.toml`

fixes #1180

Co-authored-by: Taneli Hukkinen <3275109+hukkin@users.noreply.github.com>

* fix toml meta test

* use pytest.mark.parametrize to narrow test failure

* Update tests/test_config.py

Co-authored-by: Taneli Hukkinen <3275109+hukkin@users.noreply.github.com>

Co-authored-by: Taneli Hukkinen <3275109+hukkin@users.noreply.github.com>