Drop repoze.lru on Python 3

[bochecha]

Starting with Python 3.2, the functools module grew a lru_cache function which can replace our usage of repoze.lru.

[mmerickel]

Thanks! One of the maintainers will need to do some legwork to determine if this library actually covers the required features or not.

Environment markers are the correct way to handle conditional dependencies right now. For example: https://github.com/Pylons/pyramid_debugtoolbar/blob/7c32a1fc909e182f2f26ed5cef680784ffde6263/setup.py#L77

[digitalresistor]

This should be an install_requires tag instead, not a dynamic change, this way it ends up correctly in the wheels and things work as expected.

[digitalresistor]

Move this to compat.py and import from compat.py instead of repeating this try/importerror everywhere.

[bochecha]

One of the maintainers will need to do some legwork to determine if this library actually covers the required features or not.

They both implement the same thing (an LRU cache for function results), but indeed have subtle differences in behaviour, which would be better checked by a maintainer.

One such difference that I can see from a quick glance at the repoze.lru code is that it seems to allow expiring things after a certain time (with its timeout kwarg) which functools.lru_cache doesn't support.

functools.lru_cache also supports optionally caching results by type, but not repoze.lru.lru_cache.

However the subset of the API that is used in the Pyramid code base seems to work similarly in both cases.

They also handle differently a maxsize argument set to 0 or None, but that isn't used in Pyramid either.

[mmerickel]

Thanks @bochecha, I reviewed the two codebases and could not see a compelling reason not to move forward with functools.lru_cache.