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/integration/conftest.py

@@ -36,6 +36,6 @@ def site_url():
 
 @pytest.fixture
 def store(tmp_path):
-    from wabot.sessions import SessionStore
+    from wabot import SessionStore
 
     return SessionStore(path=tmp_path / "sessions.json")

tests/integration/test_persistence.py

@@ -15,7 +15,7 @@ REATTACH_SCRIPT = textwrap.dedent(
     """
     import json, sys
     import wabot
-    from wabot.sessions import SessionStore
+    from wabot import SessionStore
 
     store = SessionStore(path=sys.argv[1])
     bot = wabot.browser(session=sys.argv[2], pacing=wabot.NoPacing(), store=store)