Make also minor PEP8 coding style fixes on modified imports.

Although nth roots of negative numbers are real for odd n, the
statistics module doesn't make use of this. Remove support for
negative roots from the private _nth_root function, which
simplifies the test suite.

patches by Upendra Kumar and Stefan Krah
speed up median by using bisect, and general speedup for Decimals using as_integer_ratio

Patch by John Mark Vandenberg.

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

(Committed on Steven's behalf)

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)

by Steven D'Aprano.