refactor: rename sessions submodule to _sessions to avoid shadowing sessions()

wabot/__init__.py defines def sessions(...) and also had a sessions.py
submodule, so wabot.sessions (attribute) resolved to the function,
shadowing the module for import wabot.sessions as m style access. This
is the same class of collision that browser.py -> _browser.py already
fixed. Renaming to _sessions.py removes the ambiguity and lets
docs/api.rst drop the :no-index: Sphinx workaround in favor of plain
autoclass directives on the public SessionRecord/SessionStore
re-exports.

tests/unit/test_api.py

@@ -3,7 +3,7 @@ from unittest.mock import MagicMock
 import pytest
 
 import wabot
-from wabot.sessions import SessionRecord, SessionStore
+from wabot import SessionRecord, SessionStore
 
 
 @pytest.fixture

tests/unit/test_sessions.py

@@ -1,6 +1,6 @@
 from datetime import datetime, timedelta, timezone
 
-from wabot.sessions import SessionRecord, SessionStore
+from wabot._sessions import SessionRecord, SessionStore
 
 
 def make_record(name="scraper1", created_at=None):
@@ -63,7 +63,7 @@ class TestSessionStore:
         assert store.get("scraper1") is not None
 
     def test_default_path_is_under_platformdirs(self):
-        from wabot.sessions import default_store_path
+        from wabot._sessions import default_store_path
 
         assert default_store_path().name == "sessions.json"
         assert "wabot" in str(default_store_path())