Pyrsistent

* NOTE! This is the last version of Pyrsistent that officially supports Python 2.X!
* Fix 186, type errors with more recent versions of MyPy. Thanks qhesz for this!
* Build and test on ARM during CI. Thanks ossdev07 for this!
* Set absolute imports for python2 compatibility. Thanks michalvi for this!

* Fix 182 moduleinit name clash.

* Fix 179 Fixed 'ignore_extra' factory parameter for pvector. Thanks ss18 for this!

* Fix 174, fix a GC traversal bug in pvector evolver C extension. Thanks till-varoquaux for finding and fixing this!
* Fix 175, pytest 5 compatibility, this is a quick fix, some more work is needed to get coverage working etc.

* Fix 172, catch all exceptions during extension build to reduce chance of corner cases that prevents installation.
* Fix 171, in PVector equality comparison don's assume that other object has a length, check before calling len.
* Fix 168, write warning about failing build of C extension directly to stderr to avoid that pip silences it.
* Fix 155, update PMapEvolver type stub to better reflect implementation.

* Fix 166, Propagate 'ignore_extra' param in hierarchy. Thanks ss18 for this!
* Fix 167, thaw typing. Thanks nattofriends for this!
* Fix 154, not possible to insert empty pmap as leaf node with transform.