Describe the bug

I have a project configured to use relative_files = True: pyca/cryptography#5982, however I'm still ending up with some absolute paths in .coverage.

To Reproduce

• Python 3.8.5
• coverage 5.5
• Other relevant package: pytest-cov 2.11.1
• Code is github.com/alex/cryptography @ d73438c46210f37831a87f7757217d97caa6004d (this is presently the goofball-coverage-best-coverage branch)

To reproduce I ran: tox -e py38 -- tests/x509/ (this runs just one subdirectory of tests, that's all it takes to reproduce)

Then I looked at .coverage with `sqlite3:

sqlite> SELECT path from file limit 10;
tests/conftest.py
/home/alex/p/cryptography/src/cryptography/hazmat/backends/openssl/backend.py
tests/x509/__init__.py
tests/x509/test_ocsp.py
tests/x509/test_x509.py
/home/alex/p/cryptography/src/cryptography/hazmat/_der.py
/home/alex/p/cryptography/src/cryptography/utils.py
tests/hazmat/__init__.py
tests/hazmat/primitives/__init__.py
tests/hazmat/primitives/fixtures_dsa.py

Expected behavior

All paths are relative to pwd, meaning the /home/alex/p/cryptography/ prefix is not present.