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.

This formally describes our policies on reporting, notification and
disclosure of security issues, and provides a detailed explanation of
our full security-response process, for reference purposes.

This is necessary for the 'utils' tests to pass.