Pando

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* Only support negotiated simplates for errors - this simplifies the error path

* Update packaging tools and packages to be somewhat modern (thanks Ivoz!)

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* Fix bug where we were preventing tracebacks from reaching either the console
or the browser.

* Return 400 from anti-CRLF-injection instead of 500

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* Fix bug in aspen.testing.pytest_fixtures required to be fixed for
aspen-python-plugins tests to work

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* Fix regressions in server.py exception handling

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* Update to a newer version of Algorithm

* Started using standalone filesystem_tree module

* Clean up test harness and test client to be more usable

* Add extensible typecasting for virtual path segments

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* Major internal refactoring to use Algorithm and py.test with fixtures

* Much-improved aspen TestClient and use it for aspen's own tests

* Switch hook filters to be algorithm/flow filters

* fixed 250 - 500 if file is treated as a path

* fixed 242 - Rename UnicodeWithParams to PathParts