doc: add FTS5 requirement for sqlite backend. Document the requirement for FTS5 if you are using the sqlite backend. Tonu Mikk reported issues with using a SQLite that was missing FTS5 support (RedHat 7 sqlite with python 3.6). Even though FTS5 has been in the sqlite amalgamation since 2015-10-14 (v3.9.0) RedHat chose to not build their package with that. This leads to a traceback when initializing the database.

try:
    # Python 3+
    from http import cookies as Cookie
    from http.cookies import CookieError, BaseCookie, SimpleCookie
    from http.cookies import _getdate as get_cookie_date
except ImportError:
    # Python 2.5-2.7
    from Cookie import CookieError, BaseCookie, SimpleCookie    # noqa: F401
    from Cookie import _getdate as get_cookie_date              # noqa: F401