fix "error processing line 1 of google-showcase-nspkg.pth"

[vchudnov-g]

Running nox showcase test suites leads to errors of the form Error processing line 1 of /usr/local/google/home/…/gapic-generator-python/.nox/showcase_alternative_templates/lib/python3.9/site-packages/google-showcase-nspkg.pth

A workaround is to go to the offending file and insert a newline after the import statement. However, I've seen the problem recur on subsequent test runs.

We should figure out where that file is coming from and fix it.

It may be originating from setuptools, since it seems to appears upon setup/installation, but I am not at all sure about that.

This affects test coverage, code hygiene, developer productivity, developer efficiency/satsifaction.