Remove use of `exec` from setup.py

[jackwotherspoon]

Remove use of exec as recommended here #906 (comment)

Will complete once https://github.com/googleapis/gapic-generator-python/blob/2ad9be28500972777cec69097be6581eaefb89e4/gapic/templates/setup.py.j2#L22-L25 has been rolled out so that a googleapis PR as template.

[jackwotherspoon]

Obsolete as setup.py is no longer present after switching to pyproject.toml