CI: Update `codecov` configuration

[nawtrey]

It would be nice if the codecov behavior was more similar to some of the larger OSS projects, such as scipy.

Specific changes to make:

• [ ] Add a patch diff
• [ ] Turn the codecov PR bot comments off
• [ ] Turn annotations off (see here)
• [ ] Merge the coverage reports
• [x] Double check that pytest is generating a report in the appropriate format so codecov can pick it up
• [ ] Double check that codecov isn't ever going to fail the CI by setting the minimum values to 1/0 for the project/patch coverage values.

[nawtrey]

It appears the code coverage is currently generating the coverage report based on a single runner's results instead of combining the reports, so that needs to be fixed.

Unrelated: Probably want to change pytest command as well. We don't need pytest-xdist, so we can remove that portion along with the -n 10 which is likely doing nothing but generating heat.