From https://docs.python.org/3/whatsnew/3.6.html#new-keywords

> async and await are not recommended to be used as variable, class,
> function or module names. Introduced by PEP 492 in Python 3.5, they
> will become proper keywords in Python 3.7. Starting in Python 3.6, the
> use of async or await as names will generate a DeprecationWarning.

By adding a warning to pycodestyle.py these future warnings and syntax
errors can be caught during static code analysis.

The await expression tests were taken from PEP-492.

https://www.python.org/dev/peps/pep-0492/#id58

Starting with Python 3.6, invalid escape sequences in string literals
are now deprecated. In a future version of Python, invalid escape
sequences will be a syntax error. While this deprecation produces a
runtime warning, it only appears if warnings are enabled and the first
time the Python source is compiled to byte code. By adding a check to
pycodestyle, projects can take advantage of static analysis to catch and
fix these future syntax errors.

For more information on the deprecation, see the Python release notes,
https://docs.python.org/3/whatsnew/3.6.html#deprecated-python-behavior

> A backslash-character pair that is not a valid escape sequence now
> generates a DeprecationWarning. Although this will eventually become a
> SyntaxError, that will not be for several Python releases.

Fixes #633

Add release notes for 2.3.1 as well

Signed-off-by: Ian Lee <IanLee1521@gmail.com>

Check for prohibited identifiers occuring to the lhs of an assignment
operator or after the 'as' keyword.

Closes #341

Closes #364

In order to better high-light the name change, and
backwards incompatibility.

This also allows dependent tools (like flake8) to
pin to `pycodestyle < 2.0` as a dependency while
transitioning to the new package.

Updated references to pep8.readthedocs.io -> pycodestyle.readthedocs.io

As per their email ‘Changes to project subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

"allow" is a transitive verb, which requires an object,
so "allow to <verb>" is ungrammatical.

See discussion in #469

This fix reverts to the parsing of user config (~/.config/pep8), then local directory
config files, and finally overrides with cli options as was the behavior back in 1.5.7