Rename and deprecate filter module to prevent shadowing of built-in keyword
Check for deprecation on import is problematic.  Rather just check that filter can be imported normally.
Fixes according to PR feedback
Do not render filters documentation twice
Ensure import of old filter namespace raises warning
Filter won't be imported twice--check for its deprecation correctly
For `filter` submodule, explicitly set warning registry location
Add debug print to failing assert
Make sure warning is raised upon import
Simplify warning test
Merge pull request #1184 from stefanv/filter_rename

Rename and deprecate filter module to prevent shadowing of built-in keyword