test: make the src package importable via pythonpath instead of package files

The four empty __init__.py under tests/ existed only so pytest would walk up
to the repo root and put it on sys.path, which a bare 'poetry run pytest'
needs to import src. Setting pythonpath = ["."] says the same thing in one
line, in the place that already configures the suite.

The 'src' entry it replaces was vestigial: nothing imports a module without
the src. prefix.

The __init__.py under src/ stay. They do a different job -- they are what
makes coverage.py descend into the package and report modules that no test
imports.

Author: marcieltorres

pyproject.toml

@@ -19,7 +19,7 @@ ruff = ">=0.15.4,<0.17.0"
 
 [tool.pytest.ini_options]
 testpaths = ["tests",]
-pythonpath = ["src",]
+pythonpath = [".",]
 
 [tool.coverage.run]
 branch = true