reraise with second argument to None is not supported.

This allows the test suite to run without errors on Windows.

Corrected to always generate the cache keys from file names with
forward slashes, not backslashes.

Thanks to simonb for the report and the initial patch.

* Using str() when Python 2 expects bytes and Python 3 Unicode
* Fixed reraise-ing syntax
* Fixed slicing of byte strings

Thanks Tim Saylor for the draft patch and Aymeric Augustin and Claude Paroz for feedback.

See PEP 3114. `next` is retained as an alias for Python 2.

The six addition has been borrowed from:
https://bitbucket.org/gutworth/six/changeset/733ef740

Fix #18062: Document best practices for choices in model fields.

In Python 3, the str type has an __iter__ attribute. Therefore, the
presence of an __iter__ attribute is not sufficient to distinguish
'standard' iterables (list, tuple) from strings.

Of course, __nonzero__ alias has been kept for Python 2 compatibility.

in the app directories template loader.

Add new security-policy documentation.