Closes issue 27921: Disallow backslashes anywhere in f-strings. This is a temporary restriction. In 3.6 beta 2, the plan is to again allow backslashes in the string parts of f-strings, but disallow them in the expression parts.

Add also a newline for readability in normalize_encoding().

Based on patch by Xiang Zhang.

Based on patch by Xiang Zhang.

Upstream https://github.com/python/asyncio/pull/408 by Ron Frederick.

Upstream https://github.com/python/asyncio/pull/374 by Ron Frederick.

Patch by Ville Skyttä.