Add a config object

- add the following file:
  + src/urban_meal_delivery/_config.py
- a config module is created holding two sets of configurations:
  + production => against the real database
  + testing => against a database with test data
- the module is "protected" (i.e., underscore) and imported at the
  top level via a proxy-like object `config` that detects in which of
  the two environments the package is being run

tests/test_config.py

@@ -0,0 +1,45 @@
+"""Test the package's configuration module."""
+
+import pytest
+
+from urban_meal_delivery import _config as config_mod  # noqa:WPS450
+
+
+envs = ['production', 'testing']
+
+
+@pytest.mark.parametrize('env', envs)
+def test_config_repr(env):
+    """Config objects have the text representation '<configuration>'."""
+    config = config_mod.get_config(env)
+
+    assert str(config) == '<configuration>'
+
+
+def test_invalid_config():
+    """There are only 'production' and 'testing' configurations."""
+    with pytest.raises(ValueError, match="'production' or 'testing'"):
+        config_mod.get_config('invalid')
+
+
+@pytest.mark.parametrize('env', envs)
+def test_database_uri_set(env, monkeypatch):
+    """Package does NOT emit warning if DATABASE_URI is set."""
+    uri = 'postgresql://user:password@localhost/db'
+    monkeypatch.setattr(config_mod.ProductionConfig, 'DATABASE_URI', uri)
+    monkeypatch.setattr(config_mod.TestingConfig, 'DATABASE_URI', uri)
+
+    with pytest.warns(None) as record:
+        config_mod.get_config(env)
+
+    assert len(record) == 0  # noqa:WPS441,WPS507
+
+
+@pytest.mark.parametrize('env', envs)
+def test_no_database_uri_set(env, monkeypatch):
+    """Package does not work without DATABASE_URI set in the environment."""
+    monkeypatch.setattr(config_mod.ProductionConfig, 'DATABASE_URI', None)
+    monkeypatch.setattr(config_mod.TestingConfig, 'DATABASE_URI', None)
+
+    with pytest.warns(UserWarning, match='no DATABASE_URI'):
+        config_mod.get_config(env)