make NominatimAPI[Async] a context manager

If close() isn't properly called, it can lead to odd error messages
about uncaught exceptions.

test/python/api/test_api_polygons_v1.py

@@ -12,19 +12,10 @@ import datetime as dt
 from pathlib import Path
 
 import pytest
-import pytest_asyncio
 
 from fake_adaptor import FakeAdaptor, FakeError, FakeResponse
 
 import nominatim_api.v1.server_glue as glue
-import nominatim_api as napi
-
-@pytest_asyncio.fixture
-async def api():
-    api = napi.NominatimAPIAsync(Path('/invalid'))
-    yield api
-    await api.close()
-
 
 class TestPolygonsEndPoint: