and backrefs can't be used inside []

Closes #3011.

* Expand comments.
* Swap variable names in the sum_exact code so that x and y
  are consistently chosen as the larger and smaller magnitude
  values respectively.

Issue 2855: Fix obscure crasher by slowing down the entire module.  Mimics what was done to dictionaries in r59223.

all the upper level libraries that use it, including urllib2.
Added and fixed some tests, and changed docs correspondingly.
Thanks to John J Lee for the patch and the pusing, :)

Works on issue #2877. Thanks Quentin Gallet-Gilles for the patch.

when the TarFile is closed.

issues with it.

- Always show warnings so they are always catchable
- Make test_os_path_walk faster by walking a less populous directory

passed in.  (The assert won't prevent this in non-debug builds).