Python 3.12 changes (#44)

* use importlib.machinery instead of imp

* python 3.12 changes

* update build matrix

* build on pull_request, not push

* add to_filename helper

* open pyproject.toml as bniary

* redirect "tomllib or tomli" through enscons.toml

* update workflow

* add pypy3.10

* edit CHANGES

* note packaging normalizers

* syntax

* require successful SCons exit code

* add python 3.8+ to workflow