The "add-path" problem is fixed at 2.0.1.
refs: https://github.com/deadsnakes/issues/issues/135

The `add-path` command that deadsnake/action internally uses is
disabled by default now [*1].  But the action is still does not
support new style environment files commands [*2].  So this enables
the deprecated command via `ACTIONS_ALLOW_UNSECURE_COMMANDS` envvar.

[*1]: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
[*2]: https://github.com/deadsnakes/issues/issues/135

Sort imports with isort

Keep imports alphabetically sorted and their order homogeneous across
Python source files.

The isort project has more feature and is more active than the
flake8-import-order plugin.

Most issues caught were simply import ordering from the same module.
Where imports were purposefully placed out of order, tag with
isort:skip.