Limit xdist distribution to files

I get regularly spurious failures in tox in the Mypy tests because of
being able to access certain files. I hope running the file
non-concurently helps here.

Author: hynek

tox.ini

@@ -34,7 +34,8 @@ set_env =
     COVERAGE_PROCESS_START={toxinidir}/pyproject.toml
     PYTHONWARNINGS=d
 commands_pre = python -c 'import pathlib; pathlib.Path("{env_site_packages_dir}/cov.pth").write_text("import coverage; coverage.process_startup()")'
-commands = coverage run -m pytest {posargs:-n auto}
+# We group xdist execution by file because otherwise the Mypy tests have race conditions.
+commands = coverage run -m pytest {posargs:-n auto --dist loadfile}
 
 [testenv:coverage-report]
 # Keep base_python in-sync with .python-version-default