Add test coverage for api.py

This change adds tests for the highlevel api that exposes the http
methods directly.

tests/test_api.py

@@ -0,0 +1,12 @@
+import pytest
+
+import requests
+
+
+@pytest.mark.parametrize(
+    "method", ("get", "head", "options", "delete", "put", "post", "patch")
+)
+def test_highlevel_api(httpbin, method):
+    function = getattr(requests, method)
+    response = function(httpbin("/status/200"))
+    assert response.status_code == 200