Add changelog items for 2.3.1...master

This adds some module level configuration points for users to define
how many blank lines they want in their code. It paves the way for
someone to develop a flake8 plugin to configure this in pycodestyle.

Fixes #732

* Add special case to maximum_line_length to ignore long shebang lines.

* Add test for ignoring long shebang lines.

* Clean up shebang line check.

Python 3.7 added a warning for a future feature of nested regular expressions. To avoid
this warning we escape what is not a nested regex. This also keeps track of the `async`
keyword and handles it appropriately.

Closes gh-728 

Add W606 warning for async and await keywords in Python 3.7

Add E252 on missing whitespace for annotated parameter defaults

Describe code W504 line break after binary operator

Cross reference changes in pull request #502 which added this.

Add W504 for line breaks before binary operators

This flips the W503 rule to enforce line breaks before binary operators.

Related #498

Fix handling of diffs with mnemonic prefixes

Add Python 3.6 to test list in dev docs

Plus Travis CI typo

Allow continuation of verbosity level

Fix space in flag name

Previously, the line break in the source would result in
`--hang- closing` (note the space) being rendered in the HTML.

Fix missed quoting in E722 check

The warning looks fairly odd:

E722 do not use bare except'

change this to

E722 do not use bare 'except'

Document all supported Python versions as trove classifiers

Enable pip cache in Travis CI

Can speed up builds and reduce load on PyPI servers.

For more information, see:

https://docs.travis-ci.com/user/caching/#pip-cache

Include license file in the generated wheel package

Use MANIFEST.in to include LICENSE in the source distribution.

The wheel package format supports including the license file. This is
done using the [metadata] section in the setup.cfg file. For additional
information on this feature, see:

https://wheel.readthedocs.io/en/stable/index.html#including-the-license-in-the-generated-wheel-file

Add examples