fix: ensure 'package_name' always uses underscores

Without this fix, entering 'my-package' as the project name results in a
directory called 'src/my-package', which is not importable in Python.

Also adds a test.

reps/templates/python/cookiecutter.json

@@ -1,7 +1,7 @@
 {
     "project_name": "My Project",
     "__project_slug": "{{cookiecutter.project_name|lower|replace(' ', '-')}}",
-    "__package_name": "{{cookiecutter.project_name|lower|replace(' ', '_')}}",
+    "__package_name": "{{cookiecutter.project_name|lower|replace(' ', '_')|replace('-', '_')}}",
     "short_description": "",
     "author": "Mozilla Release Engineering <[email protected]>",
     "github_slug": "mozilla-releng/{{cookiecutter.__project_slug}}",

test/test_template_python.py

@@ -1,3 +1,10 @@
+from pprint import pprint
+
+import pytest
+from cookiecutter.generate import generate_context
+from cookiecutter.prompt import prompt_for_config
+
+
 def test_generated_files(reps_new):
     name = "foo"
     expected = [
@@ -48,3 +55,45 @@ def test_generated_files(reps_new):
         actual.append(str(path.relative_to(project)))
 
     assert sorted(actual) == sorted(expected)
+
+
+@pytest.mark.parametrize(
+    "extra_context,expected",
+    (
+        pytest.param(
+            {"project_name": "My Package"},
+            {
+                "__project_slug": "my-package",
+                "__package_name": "my_package",
+                "short_description": "",
+                "author": "Mozilla Release Engineering <[email protected]>",
+                "github_slug": "mozilla-releng/my-package",
+                "min_python_version": "3.7",
+                "__min_tox_python_version": "37",
+                "__max_tox_python_version": "311",
+                "trust_domain": "mozilla",
+                "trust_project": "my-package",
+                "level": "1",
+                "__codecov_secrets_path": "project/mozilla/my-package/level-any/codecov",  # noqa
+                "_copy_without_render": [".github/workflows/codeql-analysis.yml"],
+            },
+            id="defaults",
+        ),
+        pytest.param(
+            {"project_name": "foo-bar"},
+            {"__package_name": "foo_bar"},
+            id="package_name_normalized",
+        ),
+    ),
+)
+def test_cookiecutter_json(project_root, extra_context, expected):
+    cookiecutter_json = (
+        project_root / "reps" / "templates" / "python" / "cookiecutter.json"
+    )
+    context = generate_context(cookiecutter_json, extra_context=extra_context)
+    config = prompt_for_config(context, no_input=True)
+    pprint(config, indent=2)
+
+    for key, val in expected.items():
+        assert key in config
+        assert config[key] == val