pytest: explicitly add pwd to pythonpath

Recent versions of pytest no longer (by default) add pwd to the
PYTHONPATH. This affects module discovery and can lead to errors like
`No module named 'tests.conftest'` when running pytest locally. This
commit modifies pyproject.toml configuring pytest's pythonpath to
include pwd by default.

pyproject.toml

@@ -14,6 +14,7 @@ exclude = '''
 
 [tool.pytest.ini_options]
 addopts = "--import-mode=append -Werror"
+pythonpath = ["."]
 
 [tool.towncrier]
     package = "omegaconf"