Bump pyrsistent from 0.15.4 to 0.15.5

Bumps [pyrsistent](https://github.com/tobgu/pyrsistent) from 0.15.4 to 0.15.5.
- [Release notes](https://github.com/tobgu/pyrsistent/releases)
- [Changelog](https://github.com/tobgu/pyrsistent/blob/master/CHANGES.txt)
- [Commits](tobgu/pyrsistent@v0.15.4...v0.15.5)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements.txt

@@ -25,7 +25,7 @@ pyramid==1.10.4
 pyramid-mailer==0.15.1
 pyramid-multiauth==0.9.0
 pyramid-tm==2.3
-pyrsistent==0.15.4
+pyrsistent==0.15.5
 python-dateutil==2.8.0
 repoze.sendmail==4.4.1
 requests==2.22.0