Log Message:
py-pyrsistent: updated to 0.15.4

0.15.4:
 * Fix a GC traversal bug in pvector evolver C extension.
 * Fix pytest 5 compatibility, this is a quick fix, some more work is needed to \ 
get coverage working etc.

0.15.3:
 * Fix catch all exceptions during extension build to reduce chance of corner \ 
cases that prevents installation.
 * Fix in PVector equality comparison don's assume that other object has a \ 
length, check before calling len.
 * Fix write warning about failing build of C extension directly to stderr to \ 
avoid that pip silences it.
 * Fix update PMapEvolver type stub to better reflect implementation.

0.15.2:
 * Propagate 'ignore_extra' param in hierarchy.
 * Fix thaw typing.
 * Fix not possible to insert empty pmap as leaf node with transform.

0.15.1:
 * Fix installation broken on Python 2.

0.15.0:
 * Python 3.4 is no longer officially supported since it is EOL since 2019-03-18.
 * Fix major improvements to type hints.
 * Fix installation fails on some Windows platforms because fallback to Python \ 
pvector does not work.