Pytest-codecov

- Adds support for Python 3.12 & 3.13
- Fixes a bug where with `pytest-cov >= 5` and setting `cov-report=` the report was not uploaded to CodeCov

Gets rid of deprecation warning
Drops 3.6 support
Adds 3.11 support

Allows git repo detection from subdirectories.
Fixes regular expression for validating repository slugs.

Treats empty environment variables the same way as unset ones to avoid forcing users to unset environment variables.

Fixes network section in codecov payload for correct source file mapping.

Minor fixes and improvements.