Set pythonpath = "./src" on pytest.ini_options

Following conventions recommended on [pytest](https://docs.pytest.org/en/stable/explanation/goodpractices.html),
setting the `pytest.ini_options.pythonpath` variable in the
pyproject.toml file avoids needed to modify `sys.path` before each
testing module begins.

Author: dwr-zroy

pyproject.toml

@@ -68,3 +68,4 @@ build-backend = "setuptools.build_meta"
 log_cli = true
 log_cli_level = 'INFO'
 testpaths = ["tests"]
+pythonpath = "./src"