Make the py test fixture more robust

[clint-lawrence]

The tests fail for me when running with a fresh clone using doit with python 3.8 on os x.

Depending on how the test virtual environment is created, there is not always a symlink with the python. naming convention. python -m venv creates only python & python3. Using virtualenv to manually create a virtual environment creates python3.8 symlink and the tests run fine.

This fix modifies the py test fixture so that it creates its own symlink in a temp directory. Then uses that temp directory as the modified PATH passed to the py launcher environment when the test runs.

[clint-lawrence]

This really only effects development. And I see now that more recent version of venv create the pythonx.y symlink, so this is probably not worth complicating the tests for.

[brettcannon]

Since you already pointed out that newer versions of Python don't have this issue I'm going to close this to keep things simple. Thanks for investigating, though!