fix black and flake8 errors in generated pytest runner.

[tellett]

Fix flake8 and black formatting issues in generated pytest driver by adding a no-op statement instead of an empty string. The previous version resulted in a file with 2 blank lines if chdir was undefined, resulting in the following issues being raised:

bazel-out/k8-fastbuild/bin/REDACTED/__test__.py:21:1: BLK100 Black would make changes.
bazel-out/k8-fastbuild/bin/REDACTED/__test__.py:21:1: W293 blank line contains whitespace
bazel-out/k8-fastbuild/bin/REDACTED/__test__.py:23:5: E303 too many blank lines (2)

[CLAassistant]

All committers have signed the CLA.

[mattem]

Thanks for the PR!

Can we add a comment around the line in the template that gets replaced? Even if it points to the comment here, it makes the noop assignment a little less mystical to someone looking at the generated code.

[tellett]

You're welcome, thanks for the library :) .

I added a comment in the generated file, and verified that it works as intended in my local branch and in my workspace.