feat: add package skeleton, constants.py and smoke tests

Task: MIL-001#3
Task: MIL-001#7

tests/unit/test_constants.py

@@ -0,0 +1,18 @@
+"""Smoke test: the package and its constants import and hold sane values."""
+
+import password_manager
+from password_manager import constants
+
+
+def test_package_has_version() -> None:
+    assert password_manager.__version__
+
+
+def test_data_file_name_is_json() -> None:
+    assert constants.DATA_FILE_NAME.endswith(".json")
+
+
+def test_password_length_bounds_are_ordered() -> None:
+    assert constants.MIN_LETTERS <= constants.MAX_LETTERS
+    assert constants.MIN_SYMBOLS <= constants.MAX_SYMBOLS
+    assert constants.MIN_DIGITS <= constants.MAX_DIGITS