rename codecov.yml to .codecov.yml

Make it a hidden file. From the [CodeCov docs][1]:

> Can I name the file .codecov.yml?
>
> Yes, you can name the file `codecov.yml` or `.codecov.yml`. However, the
> file must still be located in the repository root, `dev/`, or `.github/`
> directories

[1]: https://docs.codecov.com/docs/codecov-yaml#can-i-name-the-file-codecovyml

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah