Fix typo in RadixSort

Eliminate empty lists in ListOfListsBuilder

See merge request !24

Strip address from ctx_factory.__str__

See merge request !25

This way, pytest is able to collect tests in parallel.

Otherwise, different test slaves think they have collected different
tests because the address of the context differs.

This reverts commit 20d0e3b3.

Fix building on macOS Sierra (10.12)

The -stdlib=libc++ flag is no longer supported on Sierra (it is now the
default option). I added a version check that adds the option for macOS
versions below Sierra (10.12) and removed the option otherwise.

Use WriteOncePersistentDict instead of PersistentDict.

See merge request !23

Fix vector-typed memory maps (#203 on github)

See merge request !22

This reverts commit dcb5d701.

Prepare for Py3.7's marking 'async' a reserved keyword

See merge request !21