Bump pyrsistent from 0.15.3 to 0.15.4
Bump pyrsistent from 0.15.3 to 0.15.4

Bumps [pyrsistent](https://github.com/tobgu/pyrsistent) from 0.15.3 to 0.15.4.
- [Release notes](https://github.com/tobgu/pyrsistent/releases)
- [Changelog](https://github.com/tobgu/pyrsistent/blob/master/CHANGES.txt)
- [Commits](https://github.com/tobgu/pyrsistent/compare/v0.15.3...v0.15.4)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
mozilla-services/kinto-dist,Kinto/kinto-dist,mozilla-services/kinto-dist,Kinto/kinto-dist