tz -> zone, update iter_tzs depends on, misc changes

doc/MODULES.org

@@ -160,7 +160,7 @@ For an extensive/complex example, you can check out ~@seanbreckenridge~'s [[http
     - you might trust your original timezone, or it might just be UTC, and you want to use something more reasonable
 
    #+begin_src python
-   Policy = Literal[
+   TzPolicy = Literal[
        'keep'   , # if datetime is tz aware, just preserve it
        'convert', # if datetime is tz aware, convert to provider's tz
        'throw'  , # if datetime is tz aware, throw exception