Add CYCLE clause support for recursive CTEs

[heinrichf-cdgnm]

Converting the USING path column to Python is involved with psycopg2. psycopg3 will convert it to a list of string tuples. I tried my best to document this in the code and in the docs.

The columns the clause adds are read through cte.col, e.g. .annotate(is_cycle=cte.col.is_cycle).

Closes #96

[heinrichf-cdgnm]

Pushed some cleanup on top.

• The cycle argument is parsed into a CycleConfig in one place now, and invalid config raises instead of silently dropping the clause.
• Column names are quoted, so mixed case names work.
• is_cycle and path are no longer annotated onto the queryset by join(). They resolve through cte.col instead: .annotate(is_cycle=cte.col.is_cycle).
• Two path assertions only held on psycopg2. They compare parsed values now.
• Both doc examples were broken. Fixed, and they run on psycopg2 and psycopg 3.

[millerdev]

@heinrichf-cdgnm Thank you for the contribution! I intend to review and hope to get this merged, but have been on vacation recently and am swamped with other work currently. Will get to it as soon as I can.