exclude 'gcovr' from list of development pip packages

[jameslamb]

Description

This project currently lists gcovr (https://pypi.org/project/gcovr/) as a pip dependency for development. I strongly suspect that that was unintentional... it doesn't look like it has any reliance on getting that package via pip (just conda, in the C++ test jobs and for local C++ development).

This proposes removing gcovr from the list of pip dependencies, so it won't get installed in the DLFW images or other places where rapids-make-pip-env from https://github.com/rapidsai/devcontainers is called.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[jameslamb]

Thanks for joining and reviewing @vyasr !

[jameslamb]

/merge