Issue #20478: avoid special casing Counter in statistics

Passing Counter objects to the Counter constructor is
special cased, going through iter() firsts ensures they
are handled the same way as any other iterable.

(Committing on Steven's behalf as I don't believe his
SSH key is registered yet)
Issue #20167: Suppress 3.4 specific 'Exception ignored' messages.
Original patch by Tal Einat.
Close #20481: Disallow mixed type input in statistics

The most appropriate coercion rules are not yet clear, so simply
disallowing mixed type input for 3.4.

(Committed on Steven's behalf)
Issue #20538: UTF-7 incremental decoder produced inconsistant string when
input was truncated in BASE64 section.
Issue #20538: UTF-7 incremental decoder produced inconsistant string when
input was truncated in BASE64 section.