add support for python-version-file (#336)

* add support for python-version-file

* Update action.yml

* update to v4, remove python-version default

* python-version overrides python-version-file, like setup-node
* checks '.python-version' by default if nothing else specified

* update tests, update to checkout@v3

* update build

* appease the linter

* remove old test for default python version

* revert readme changes

* update build