Upload code coverage results to codecov.com

[mincong-h]

Description

Upload code coverage results to https://about.codecov.io/ so that users can better understand the quality of the project and maintainers can easily track the diff for each PR.

References

• https://github.com/codecov/codecov-action
• https://coverage.readthedocs.io/en/latest/cmd.html#cmd-xml