Pytest-codecov

This adds rudimentary support for uploading JUnit XML files along with the coverage results.

`pytest-codecov` will automatically grab the junit file when generated via` --junit-xml/--junitxml`.

If you don't want that to happen you can use `--codecov-exclude-junit-xml`.

- Fixes Codecov storage endpoint

- 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.