Add constants.py and its test

Task: MIL-001#3

tests/conftest.py

@@ -0,0 +1,16 @@
+"""Shared pytest configuration for the weather data tests."""
+
+from pathlib import Path
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def isolated_working_directory(tmp_path: Path, monkeypatch: pytest.MonkeyPatch) -> None:
+    """Run every test in an empty temporary working directory.
+
+    The export has a default file name relative to the working directory. Without
+    this fixture, a test that uses the default would write into the folder pytest
+    was started from.
+    """
+    monkeypatch.chdir(tmp_path)