ci: editable-install the package in Scheduled Tests (#129)

The thin-binder refactor moved every conversion into the compiled `_rust`
extension, which maturin builds into site-packages — not the source tree.
Scheduled Tests ran tox (`unittest discover` from the repo root) without
first building the extension, so `import num2words2` resolved to the source
dir, found no `_rust`, and every test module failed to import. Add
`pip install -e .` (matching ci.yml) so the extension is present.

Author: jqueguiner

.github/workflows/scheduled-test.yml

@@ -29,6 +29,10 @@ jobs:
         python -m pip install --upgrade pip
         pip install tox tox-gh-actions
         pip install -r tests/requirements-test.txt
+        # Build the compiled `_rust` extension into the source tree so tox's
+        # `unittest discover` (run from the repo root) imports it instead of
+        # shadowing the package with the source dir, which has no _rust.
+        pip install -e .
 
     - name: Run tests with tox
       run: tox